GitTrustedTimestamps/hooks/timestamping

#!/bin/bash
#
#    RFC3161 and RFC5816 Timestamping for git repositories.
#
#    Copyright (c) 2021 Mabulous GmbH
#    Authors: Matthias Bühlmann
#
#    This program is free software: you can redistribute it and/or modify
#    it under the terms of the GNU Affero General Public License as
#    published by the Free Software Foundation, either version 3 of the
#    License, or (at your option) any later version.
#
#    This program is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty of
#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#    GNU Affero General Public License for more details.
#
#    You should have received a copy of the GNU Affero General Public License
#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
#    
#    The interactive user interfaces in modified source and object code versions
#    of this program must display Appropriate Legal Notices, as required under
#    Section 5 of the GNU Affero General Public License version 3. In accordance
#    with Section 7(b) of the GNU Affero General Public License, you must retain
#    the Info line in every timestamp that is created or manipulated using a
#    covered work.
#    
#    You can be released from the requirements of the license by purchasing
#    a commercial license. Buying such a license is mandatory as soon as you
#    develop commercial activities involving this software without
#    disclosing the source code of your own applications.
#    These activities include: offering paid services to customers as an ASP,
#    providing data storage and archival services, shipping this software with a 
#    closed source product.
#    
#    For more information, please contact Mabulous GmbH at this
#    address: info@mabulous.com
#

declare -r TMP_DIR="$(mktemp -d)"
mkdir -p "$TMP_DIR"/ltvdir/certs
mkdir -p "$TMP_DIR"/ltvdir/crls
declare -r TMP_LTV_DIR="$TMP_DIR"/ltvdir

#set exit trap to clean up temporary files
exit_trap() {
  local -i EXIT_CODE="$?"
  rm -rf -- "$TMP_DIR"
  exit "$EXIT_CODE"
}
trap "exit_trap" EXIT

declare OUT_STREAM=/dev/null
#uncomment for verbose output
#OUT_STREAM=/dev/stdout

#echo red text
echo_error() {
  local RED='\033[0;31m'
  local NO_COLOR='\033[0m'
  echo -e "${RED}$1${NO_COLOR}"
}

#echo yellow text
echo_warning() {
  local YELLOW='\033[1;33m'
  local NO_COLOR='\033[0m'
  echo -e "${YELLOW}$1${NO_COLOR}"
}

#echo light blue text
echo_info() {
  local LIGHT_BLUE='\033[1;34m'
  local NO_COLOR='\033[0m'
  echo -e "${LIGHT_BLUE}$1${NO_COLOR}"
}

#echo dark gray text to OUT_STREAM
log() {
  local DARK_GRAY='\033[1;30m'
  local NO_COLOR='\033[0m'
  echo -e "${DARK_GRAY}$1${NO_COLOR}" > "$OUT_STREAM"
}

#assertion
#param 1: condition as string
#param 2: message
assert() {
  local -r CONDITION="$1"
  local MESSAGE="$2"

  if [ -z "$MESSAGE" ]; then
    MESSAGE="$CONDITION"
  fi
  local -r STACK_DEPTH=${#BASH_SOURCE[@]}
  local -r BACKTRACE="for ((i=1; i<$STACK_DEPTH; i++)); do
                        echo_error "\"'  [$i]: ${BASH_SOURCE[$i]} : ${FUNCNAME[$i]} line ${BASH_LINENO[$i-1]}'\""
                      done"

  local -r ASSERTION="if $CONDITION; then
                        :
                      else
                        echo_error "\""Assertion failed: $MESSAGE"\""
                        echo_error "\""Backtrace:"\""
                        $BACKTRACE
                        exit 1
                      fi"
  if ! eval "$ASSERTION"; then
    echo_info "$ASSERTION"
    echo_error "Assertion in ${BASH_SOURCE[0]} on line ${BASH_LINENO[0]} is malformed."
    exit 1
  fi
}

declare -r TOKEN_HEADER="-----BEGIN RFC3161 TOKEN-----"
declare -r TOKEN_FOOTER="-----END RFC3161 TOKEN-----"
declare -r SUBJECT_LINE="-----TIMESTAMP COMMIT-----"
declare -r -i TIMESTAMPING_VERSION=1
declare -r TRAILER_TOKEN_VERSION="Version:"
declare -r TRAILER_TOKEN_PREIMAGE="Preimage:"
declare -r TRAILER_TOKEN_ALGO="Algorithm:"
declare -r TRAILER_TOKEN_DIGEST="Digest:"
declare -r TRAILER_TOKEN_TIMESTAMP="Timestamp:"
#get hashing algorithm used by repo
declare -r ALGO=$(git rev-parse --show-object-format)
#get length of hashes of the used algorithm
declare -r -i HASH_LENGTH=$(printf "0" | openssl dgst -"$ALGO" -binary | xxd -p -c 256 | awk '{print length; exit}')
#get directory to store validation data
declare -r ROOT_DIR=$(git rev-parse --git-dir)/..
declare -r LTV_DIR="$ROOT_DIR"/.timestampltv
#get directory for trusted RootCA certificates
declare -r CA_PATH=$(git rev-parse --git-path hooks/trustanchors)
if [ ! -d "$CA_PATH" ]; then
  mkdir -p "$CA_PATH"
fi

# function to assemble the string that is used to create the digest to be timestamped
# param1: hash of parent commit
# param2: hash of tree
# param2: OUT variable, the string used to compute digest
get_preimage_from_tree_and_parent() {
  local -r TREE_HASH="$1"
  local -r PARENT_COMMIT_HASH="$2"
  local -n PREIMAGE_OUT="$3"
  log "get_preimage_from_tree_and_parent for tree $TREE_HASH and commit $PARENT_COMMIT_HASH"

  #perform precondition checks
  assert "[ ${#TREE_HASH} -eq $HASH_LENGTH ]" "Precondition: hash $TREE_HASH must have length $HASH_LENGTH."
  assert "[ ${#PARENT_COMMIT_HASH} -eq $HASH_LENGTH ]" "Precondition: hash $PARENT_COMMIT_HASH must have length $HASH_LENGTH."

  PREIMAGE_OUT="version:${TIMESTAMPING_VERSION,,},parent:${PARENT_COMMIT_HASH,,},tree:${TREE_HASH,,}"
}

# function to compute digest that is timestamped, which is a hash that depends on commit hash and tree hash
# param1: hash of parent commit
# param2: hash of tree
# param2: OUT variable, the DIGEST
compute_digest_from_tree_and_parent() {
  local -r TREE_HASH="$1"
  local -r PARENT_COMMIT_HASH="$2"
  local -n DIGEST_OUT="$3"
  log "compute_digest_from_tree_and_parent for tree $TREE_HASH and commit $PARENT_COMMIT_HASH"

  #perform precondition checks
  assert "[ ${#TREE_HASH} -eq $HASH_LENGTH ]" "Precondition: hash $TREE_HASH must have length $HASH_LENGTH."
  assert "[ ${#PARENT_COMMIT_HASH} -eq $HASH_LENGTH ]" "Precondition: hash $PARENT_COMMIT_HASH must have length $HASH_LENGTH."

  local PREIMAGE
  get_preimage_from_tree_and_parent "$TREE_HASH" "$PARENT_COMMIT_HASH" PREIMAGE
  DIGEST_OUT=$(printf "%s" "$PREIMAGE" | openssl dgst -"$ALGO" -binary | xxd -p -c 256)

  #perform postcondition checks
  assert "[ ${#DIGEST_OUT} -eq $HASH_LENGTH ]" "Postcondition: hash $DIGEST_OUT must have length $HASH_LENGTH."
}

# function to extract timestamp token from commit. The extracted token are saved to files in the folder provided
# and are named *.extracted_token.tst where * is the (0-based) index of the token as ordered within the commit.
# param1: hash of commit to extract token from
# param2: path to dir into which the token should be saved
# param3: OUT variable containing the version of the timestamp commit (0 if no version is defined but timestamp token were found, -1 if this commit does not contain any timestamp token)
# param4: OUT array containing the tsa urls from which each timestamp was retrieved
# param5: OUT array containing the paths to the extracted timestamp tokens
extract_token_from_commit() {
  local -r COMMIT_HASH="$1"
  local -r TOKEN_DIR="$2"
  local -n VERSION_OUT="$3"
  local -n URL_ARRAY_OUT="$4"
  local -n TOKEN_ARRAY_OUT="$5"
  log "extract_token_from_commit $COMMIT_HASH into $TOKEN_DIR"

  #perform precondition checks
  assert "[ ${#COMMIT_HASH} -eq $HASH_LENGTH ]" "Precondition: hash $COMMIT_HASH must have length $HASH_LENGTH."
  assert "[ -d $TOKEN_DIR ]" "Precondition: directory $TOKEN_DIR must exist."
  local -r COMMIT_CONTENT=$(git cat-file -p "$COMMIT_HASH")
  assert "[ ${#COMMIT_CONTENT} -gt 0 ]" "Precondition: $COMMIT_HASH must be a valid commit hash"

  #remove files from possible previous runs
  rm -f "$TMP_DIR"/*.extracted_token.pem
  rm -f "$TMP_DIR"/*.extracted_token.url
  #extract timestamp tokens
  local URL_START=${#TRAILER_TOKEN_TIMESTAMP}
  URL_START=$(( $URL_START + 2))
  local -i NUM_EXTRACTED=$(printf "%s" "$COMMIT_CONTENT" | awk '$0~trailerregex{ i++; insidetimestamp=1; print substr($0,urlstart) > tmpdir i ".extracted_token.url" } /-----END/{insidepem=0; insidetimestamp=0} insidepem{print substr($0,2) > tmpdir i ".extracted_token.pem"} insidetimestamp && /-----BEGIN/{insidepem=1} END {print i}' tmpdir="$TMP_DIR/" trailerregex="^$TRAILER_TOKEN_TIMESTAMP" urlstart="$URL_START")
  if [ -z "$NUM_EXTRACTED" ]; then
    NUM_EXTRACTED=0
  fi

  local -r TMP_DER="$TMP_DIR"/extracted_token.der
  local -i IDX=0;
  for (( i=1; i<=$NUM_EXTRACTED; i++ )); do
    local EXTRACTED_PEM_FILE="$TMP_DIR"/"$i".extracted_token.pem
    local EXTRACTED_TOKEN="$TOKEN_DIR"/"$IDX".extracted_token.tst
    openssl base64 -d -in "$EXTRACTED_PEM_FILE" -out "$TMP_DER"
    #since a commit might contain some unrelated trailer with the name "Timestamp:" that also contains PEM header and footer, non-timestamp-tokens should be skipped
    if ! openssl ts -reply -token_in -token_out -in "$TMP_DER" -out "$EXTRACTED_TOKEN" &> "$OUT_STREAM"; then
      echo_warning "A PEM encoded trailer labeled $TRAILER_TOKEN has been found in commit $COMMIT_HASH which does not seem to be a timestamp token. Skipping."
      continue
    fi
    local TSA_URL=$(cat "$TMP_DIR"/"$i".extracted_token.url)
    URL_ARRAY_OUT+=("$TSA_URL")
    TOKEN_ARRAY_OUT+=("$EXTRACTED_TOKEN")
    ((IDX++))
  done

  local VERSION_START=${#TRAILER_TOKEN_VERSION}
  VERSION_START=$(( $VERSION_START + 1))
  VERSION_OUT=$(($(printf "%s" "$COMMIT_CONTENT" | awk '$0~trailerregex{print substr($0,versionstart); exit}' trailerregex="^$TRAILER_TOKEN_VERSION" versionstart="$VERSION_START")))
  if [ -z "$VERSION_OUT" ]; then
    VERSION_OUT=0
  fi
  if [ $IDX -eq 0 ]; then
    VERSION_OUT=-1
  fi

  #perform postcondition checks
  assert "[ ${#TOKEN_ARRAY_OUT[@]} -eq ${#URL_ARRAY_OUT[@]} ]" "Postcondition: Arrays must have equal length."
  assert "[ $IDX -gt 0 ] || [ $VERSION_OUT -eq -1 ]" "Postcondition: Version must be -1 if it does not contain token."
  assert "[ $IDX -eq 0 ] || [ $VERSION_OUT -ge 0 ]" "Postcondition: Version must be 0 or greater if token were extracted."
}

# function to extract ESSCertID or ESSCertIDv2 of TSA from token
# param1: path to token in DER encoding
# param2: OUT variable, the ID
get_tsa_cert_id() {
  local TOKEN_FILE="$1"
  local -n CERT_ID_OUT="$2"
  log "get_tsa_cert_id for $TOKEN_FILE"

  #perform precondition checks
  assert "[ ! -z $TOKEN_FILE ]" "Precondition: Path to token file must not be empty."
  assert "[ -f $TOKEN_FILE ]" "Precondition: Token file $TOKEN_FILE must exist."

  #this works for both ESSCertID as well as ESSCerrtIDv2 since the version2 identifier is id-smime-aa-signingCertificateV2
  CERT_ID_OUT=$(openssl asn1parse -inform DER -in "$TOKEN_FILE" \
            | awk '/:id-smime-aa-signingCertificate/{f=1} f && /\[HEX DUMP\]:/ {print; exit}' \
            | sed 's/^.*\[HEX DUMP\]://1')

  #perform postcondition checks
  assert "[ ! -z $CERT_ID_OUT ]" "Postcondition: Token in file $TOKEN_FILE must contain ESSCertID or ESSCertIDv2"
}

# function to extract hashing algorithm used in the ESSCertID (always sha1) or ESSCertIDv2
# param1: path to token in DER encoding
# param2: OUT variable, the hashing algorithm string
get_cert_id_hash_agorithm() {
  local TOKEN_FILE="$1"
  local -n ALGO_NAME_OUT="$2"
  log "get_cert_id_hash_agorithm for $TOKEN_FILE"

  #perform precondition checks
  assert "[ ! -z $TOKEN_FILE ]" "Precondition: Path to token file must not be empty."
  assert "[ -f $TOKEN_FILE ]" "Precondition: Token file $TOKEN_FILE must exist."

  local PARSED=$(openssl asn1parse -inform DER -in "$TOKEN_FILE")

  if [[ "$PARSED" == *":id-smime-aa-signingCertificateV2"* ]]; then
    #TODO: extract non-default hashing algorithms
    ALGO_NAME_OUT="sha256"
  elif [[ "$PARSED" == *":id-smime-aa-signingCertificate"* ]]; then
    ALGO_NAME_OUT="sha1"
  else
    ALGO_NAME_OUT="unknown"
  fi

  #perform postcondition checks
  assert "[ $ALGO_NAME_OUT != 'unknown' ]" "Postcondition: Token in file $TOKEN_FILE must contain ESSCertID or ESSCertIDv2"
}

# function to extract digest from token file
# param1: path to token in DER encoding
# param2: OUT variable, the digest
get_token_digest() {
  local TOKEN_FILE="$1"
  local -n DIGEST_OUT="$2"
  log "get_token_digest for $TOKEN_FILE"

  #perform precondition checks
  assert "[ ! -z $TOKEN_FILE ]" "Precondition: Path to token file must not be empty."
  assert "[ -f $TOKEN_FILE ]" "Precondition: Token file $TOKEN_FILE must exist."

  local OFFSET=$(openssl asn1parse -inform DER -in "$TOKEN_FILE" \
                 | awk '/:id-smime-ct-TSTInfo/{f=1} f && /\[HEX DUMP\]:/ {print; exit}' \
                 | sed 's/:.*//' | sed 's/^[ \t]*//')
  DIGEST_OUT=$(openssl asn1parse -inform DER -in "$TOKEN_FILE" -strparse "$OFFSET" \
           | awk '/\[HEX DUMP\]:/ {print; exit}' \
           | sed 's/^.*\[HEX DUMP\]://1')

  #perform postcondition checks
  assert "[ ${#DIGEST_OUT} -eq $HASH_LENGTH ]" "Postcondition: hash $DIGEST_OUT must have length $HASH_LENGTH."
}

# function to extract unix time of timestamp from token file
# param1: path to token in DER encoding
# param2: OUT variable, the unix time
get_token_unix_time() {
  local TOKEN_FILE="$1"
  local -n UNIXTIME="$2"
  log "get_token_unix_time for $TOKEN_FILE"

  #perform precondition checks
  assert "[ ! -z $TOKEN_FILE ]" "Precondition: Path to token file must not be empty."
  assert "[ -f $TOKEN_FILE ]" "Precondition: Token file $TOKEN_FILE must exist."

  local TOKEN_TIMESTAMP=$(openssl ts -reply -in "$TOKEN_FILE" -token_in -token_out -text 2> "$OUT_STREAM" \
                          | awk '/Time stamp:/{f=1} f {print; exit}' \
                          | sed 's/^.*Time stamp: //1')
  UNIXTIME=$(date "+%s" -d "$TOKEN_TIMESTAMP")

  #perform postcondition checks
  assert "[ ! -z $UNIXTIME ]" "Postcondition: Token in file $TOKEN_FILE must contain valid timestamp"
}

#function to request a timestamp for a specified digest
# param1: tsa url
# param2: digest
# param3: whether to request certificates to be included (true or false)
# param4: the file to output the token to
request_token() {
  local TSA_URL="$1"
  local DIGEST="$2"
  local REQUEST_CERTS="$3"
  local OUTPUT_FILE="$4"
  log "request_token for digest $DIGEST from url $TSA_URL. REQUEST_CERTS=$REQUEST_CERTS"

  #perform precondition checks
  assert "[ ${#DIGEST} -eq $HASH_LENGTH ]" "Precondition: digest $DIGEST must have length $HASH_LENGTH."

  local CONTENT_TYPE="Content-Type: application/timestamp-query"
  local ACCEPT_TYPE="Accept: application/timestamp-reply"

  local REQ_FILE="$TMP_DIR"/token_req.tsq
  if [ "$REQUEST_CERTS" = true ]; then
    if ! openssl ts -query -cert -digest "$DIGEST" -"$ALGO" -out "$REQ_FILE" &> "$OUT_STREAM"; then
      echo "Error: Failed to create token query"
      return 1
    fi
  else
    if ! openssl ts -query -digest "$DIGEST" -"$ALGO" -out "$REQ_FILE" &> "$OUT_STREAM"; then
      echo "Error: Failed to create token query"
      return 1
    fi
  fi
  local RESPONSE_FILE="$TMP_DIR"/response.tsr
  if ! curl "$TSA_URL" -H "$CONTENT_TYPE" -H "$ACCEPT_TYPE" --data-binary @"$REQ_FILE" --output "$RESPONSE_FILE" &> "$OUT_STREAM"; then
    echo "Error: Failed to get response from $TSA_URL"
    return 1
  fi
  local RESPONSE_STATUS=$(openssl ts -reply -in "$RESPONSE_FILE" -text 2> "$OUT_STREAM" | awk '/Status: /{print; exit}' | sed 's/Status: //' | sed 's/\.//')
  if [ "$RESPONSE_STATUS" != "Granted" ]; then
    echo "Error: Token request was not granted."
    if [ -z "$RESPONSE_STATUS" ]; then
      cat "$RESPONSE_FILE"
      echo ""
    else
      local STATUS_INFO=$(openssl ts -reply -in "$RESPONSE_FILE" -text 2> "$OUT_STREAM" | awk '/Status info:/{f=1} f {print} /Failure info: /{exit}')
      echo "$STATUS_INFO"
      echo "Note: If rejection reason is unrecognized or unsupported algorithm, then this tsa cannot be used for this repository, since it uses --object-format=$ALGO"
      echo "The token request was:"
      openssl ts -query -in "$REQ_FILE" -text 2> "$OUT_STREAM"
    fi
    return 1
  fi
  if ! openssl ts -reply -in "$RESPONSE_FILE" -token_out -out "$OUTPUT_FILE" &> "$OUT_STREAM"; then
    echo "Error: Not a valid TSA response in file $RESPONSE_FILE"
    return 1
  fi
}

#builds a certificate chain for token. The passed token must have been requested with -cert option
# and with matching digest.
# param1: token file. Token must have been requested with -cert option and with digest of param2
# param2: the digest the token was requested for
# param3: the tsa url from which the token was requested
# param4: the output file for the chain. It contains all certificates in order, with the first
#         one being the TSA cetificate and the last one the self-signed root certificate.
build_certificate_chain_for_token() {
  local TOKEN_FILE="$1"
  local DIGEST="$2"
  local TSA_URL="$3"
  local OUT_CERT_FILE="$4"
  log "build_certificate_chain_for_token for token $TOKEN_FILE from $TSA_URL with digest $DIGEST and store in certificate file $OUT_CERT_FILE"

  #perform precondition checks
  assert "[ ! -z $TOKEN_FILE ]" "Precondition: Path to token file must not be empty."
  assert "[ -f $TOKEN_FILE ]" "Precondition: Token file $TOKEN_FILE must exist."
  assert "[ ${#DIGEST} -eq $HASH_LENGTH ]" "Precondition: digest $DIGEST must have length $HASH_LENGTH."

  local DUMMY_TOKEN="$TMP_DIR"/dummy_token.tst
  local ALL_EXTRACTED_CERTS="$TMP_DIR"/extracted_certs.pem
  local CHAIN=()
  #if the TSA uses multiple certificates to sign tokens it may take a few attempts to get one containint the proper signer
  #TODO: allow to set maximum retry attempts
  local SIGNING_CERT_ID=""
  get_tsa_cert_id "$TOKEN_FILE" SIGNING_CERT_ID
  local CERT_ID_HASH_ALGO=""
  get_cert_id_hash_agorithm "$TOKEN_FILE" CERT_ID_HASH_ALGO
  for i in {1..10} ;do
    #request dummy token. Use current commit digest
    request_token "$TSA_URL" "$DIGEST" true "$DUMMY_TOKEN"

    #extract certifcates
    openssl pkcs7 -inform DER -in "$DUMMY_TOKEN" -print_certs -outform PEM -out "$ALL_EXTRACTED_CERTS" &> "$OUT_STREAM"

    #remove files from previous runs
    rm -f "$TMP_DIR"/*.extracted.pem.cer
    rm -f "$TMP_DIR"/cert_chain_*.pem.cer

    #extract all individual certificates from ALL_EXTRACTED_CERTS
    cat "$ALL_EXTRACTED_CERTS" \
    | awk '/-----BEGIN CERTIFICATE-----/ { i++; } /-----BEGIN CERTIFICATE-----/, /-----END CERTIFICATE-----/ \
    { print > tmpdir i ".extracted.pem.cer" }' tmpdir="$TMP_DIR/"

    #find cetificate that signed token
    while read EXTRACTED_CERT; do
      local CERT_ID=$(openssl x509 -inform PEM -in "$EXTRACTED_CERT" -outform DER | openssl dgst -"$CERT_ID_HASH_ALGO" -binary | xxd -p -c 256)
      #if openssl ts -verify -digest "$DIGEST" -in "$TOKEN_FILE" -token_in -partial_chain -CAfile "$EXTRACTED_CERT" &> "$OUT_STREAM"; then
      if [ "${SIGNING_CERT_ID,,}" == "${CERT_ID,,}" ]; then
        #found the signer certificate
        CHAIN+=("$TMP_DIR"/cert_chain_"${#CHAIN[@]}".pem.cer)
        mv -f "$EXTRACTED_CERT" "${CHAIN[-1]}"
        break 2
      fi
    done <<< $(ls "$TMP_DIR"/*.extracted.pem.cer)
  done
  
  if [ ${#CHAIN[@]} -eq 0 ]; then
    echo "Unable to download token that contains signing cert for this token:"
    openssl ts -reply -token_in -token_out -in "$TOKEN_FILE" -text
    return 1
  fi

  #iterate until self-signed certificate is reached
  while ! openssl verify -CAfile "${CHAIN[-1]}" "${CHAIN[-1]}" &> "$OUT_STREAM"; do
    #try to find parent certificate in extracted certs
    if ls "$TMP_DIR"/*.extracted.pem.cer &> "$OUT_STREAM"; then
      while read EXTRACTED_CERT; do
        if openssl verify -partial_chain -CAfile "$EXTRACTED_CERT" "${CHAIN[-1]}" &> "$OUT_STREAM"; then
          CHAIN+=("$TMP_DIR"/cert_chain_"${#CHAIN[@]}".pem.cer)
          mv -f "$EXTRACTED_CERT" "${CHAIN[-1]}"
          continue 2
        fi
      done <<< $(ls "$TMP_DIR"/*.extracted.pem.cer)
    fi 
    
    #otherwise try to find in trust store
    if ls "$CA_PATH"/*.0 &> "$OUT_STREAM"; then
      while read TRUSTED_CERT; do
        if openssl verify -partial_chain -CAfile "$TRUSTED_CERT" "${CHAIN[-1]}" &> "$OUT_STREAM"; then
          CHAIN+=("$TRUSTED_CERT")
          continue 2
        fi
      done <<< $(ls "$CA_PATH"/*.0)
    fi
    
    #otherwise try to download
    local URL=$(openssl x509 -inform PEM -noout -text -in "${CHAIN[-1]}" \
                | awk '/Authority Information Access:/{f=1} f && /CA Issuers - URI:/ {print; exit}' \
                | sed 's/^.*CA Issuers - URI://1')
    if [ -z "$URL" ]; then
      echo "Certificate ${CHAIN[-1]} does not contain Authority Information Access extension with CA issuer URL. Can't build certificate chain."
      return 1
    fi
    CHAIN+=("$TMP_DIR"/cert_chain_"${#CHAIN[@]}".pem.cer)
    local TMP_DOWNLOAD="$TMP_DIR"/tmp_download.crt
    if ! curl "$URL" --output "$TMP_DOWNLOAD" &> "$OUT_STREAM"; then
      echo "Failed to download issuer certificate from $URL"
      return 1
    fi
    #convert from DER to PEM if necessary
    if openssl x509 -inform PEM -in "$TMP_DOWNLOAD" -noout &> "$OUT_STREAM"; then
      openssl x509 -inform PEM -in "$TMP_DOWNLOAD" -outform PEM -out "${CHAIN[-1]}"
    elif openssl x509 -inform DER -in "$TMP_DOWNLOAD" -noout &> "$OUT_STREAM"; then
      openssl x509 -inform DER -in "$TMP_DOWNLOAD" -outform PEM -out "${CHAIN[-1]}"
    else
      echo "Unknown certificate file format for $URL"
      return 1
    fi
  done

  echo -n > "$OUT_CERT_FILE"
  for CERT in "${CHAIN[@]}"; do
    openssl x509 -in "$CERT" -noout -subject >> "$OUT_CERT_FILE"
    echo '' >> "$OUT_CERT_FILE"
    openssl x509 -in "$CERT" -noout -issuer >> "$OUT_CERT_FILE"
    echo '' >> "$OUT_CERT_FILE"
    cat "$CERT" >> "$OUT_CERT_FILE"
    echo '' >> "$OUT_CERT_FILE"
  done
}

# Tries to download CRLs for the entire chain and store them together in PEM encoding in an output file.
# param1: path to the certificate chain in PEM format
# param2: path to output file
# TODO: performance of this could be improved by using OCSPs to check for changes first
download_crls_for_chain() {
  local CERT_FILE="$1"
  local OUTPUT_FILE="$2"
  log "download_crls_for_chain for certificate file $CERT_FILE and store to $OUTPUT_FILE"

  #perform precondition checks
  assert "[ ! -z $CERT_FILE ]" "Precondition: Path to certificate file must not be empty."
  assert "[ -f $CERT_FILE ]" "Precondition: Certificate file $CERT_FILE must exist."

  echo -n > "$OUTPUT_FILE"
  local CRL_TMP="$TMP_DIR"/crl_tmp.crl
  #remove files from possible previous runs
  rm -f "$TMP_DIR"/*.extracted.pem.cer
  #extract all contained certificates into separate files
  cat "$CERT_FILE" \
  | awk '/-----BEGIN CERTIFICATE-----/ { i++; } /-----BEGIN CERTIFICATE-----/, /-----END CERTIFICATE-----/ \
  { print > tmpdir i ".extracted.pem.cer" }' tmpdir="$TMP_DIR/"
  
  #iterate over certificates. Ignore self-signed certificates
  ls "$TMP_DIR"/*.extracted.pem.cer | while read EXTRACTED_CERT; do
    if ! openssl verify -CAfile "$EXTRACTED_CERT" "$EXTRACTED_CERT" &> "$OUT_STREAM"; then
      local URL=$(openssl x509 -inform PEM -in $EXTRACTED_CERT -text -noout \
                  | awk '/CRL Distribution Points:/{f=1} f && /URI:/ {print; exit}' \
                  | sed 's/^.*URI://1')
      if curl "$URL" --output "$CRL_TMP" &> "$OUT_STREAM"; then
        if openssl crl -in "$CRL_TMP" -inform DER -noout &> "$OUT_STREAM"; then
          openssl crl -in "$CRL_TMP" -inform DER >> "$OUTPUT_FILE"
        elif openssl crl -in "$CRL_TMP" -inform PEM -noout &> "$OUT_STREAM"; then
          openssl crl -in "$CRL_TMP" -inform PEM >> "$OUTPUT_FILE"
        else
          echo "Unknown CRL file format for $URL"
          return 1    
        fi
      else
        echo "Failed to download CRL from $URL"
        return 1
      fi  
    fi
  done
}

# Check whether the file containing the certificates to verify the token are available and if not,
# request and add them to the commit.
# param1: path to token in DER encoding
# param2: digest to verify
# param3: tsa url
verify_token_and_add_ltv_data() {
  local TOKEN_FILE="$1"
  local DIGEST="$2"
  local TSA_URL="$3"
  log "verify_token_and_add_ltv_data for token $TOKEN_FILE and digest $DIGEST from url $TSA_URL"

  #perform precondition checks
  assert "[ ! -z $TOKEN_FILE ]" "Precondition: Path to token file must not be empty."
  assert "[ -f $TOKEN_FILE ]" "Precondition: Token file $TOKEN_FILE must exist."
  assert "[ ${#DIGEST} -eq $HASH_LENGTH ]" "Precondition: digest $DIGEST must have length $HASH_LENGTH."

  local SIGNING_CERT_ID
  get_tsa_cert_id "$TOKEN_FILE" SIGNING_CERT_ID
  local CERT_CHAIN_FILE="$LTV_DIR"/certs/"$SIGNING_CERT_ID".cer
  if [ ! -f "$CERT_CHAIN_FILE" ]; then
    CERT_CHAIN_FILE="$TMP_LTV_DIR"/certs/"$SIGNING_CERT_ID".cer
    #try to build full chain.
    if ! build_certificate_chain_for_token "$TOKEN_FILE" "$DIGEST" "$TSA_URL" "$CERT_CHAIN_FILE"; then
      echo "Unable to build certificate chain."
      return 1
    fi
    if ! openssl verify --CApath "$CA_PATH" -untrusted "$CERT_CHAIN_FILE" "$CERT_CHAIN_FILE" &> "$OUT_STREAM"; then
      echo "TSA certificate from $TSA_URL is not trusted. Check your trustanchors in $CA_PATH"
      return 1
    fi
  fi
  #verify token and download CRL data
  local CRL_CHAIN_FILE="$TMP_LTV_DIR"/crls/"$SIGNING_CERT_ID.crl"
  #only download CRL data if it hasn't been already in a previous step
  if [ ! -f "$CRL_CHAIN_FILE" ]; then
    if ! download_crls_for_chain "$CERT_CHAIN_FILE" "$CRL_CHAIN_FILE"; then
      echo "Could not download CRL data for $TOKEN_FILE"
      return 1
    fi
  fi
  #verify signing certificate
  local TOKEN_UNIXTIME=''
  get_token_unix_time "$TOKEN_FILE" TOKEN_UNIXTIME
  #validate signing certificate
  if ! openssl verify -attime "$TOKEN_UNIXTIME" -CApath "$CA_PATH" -CRLfile "$CRL_CHAIN_FILE" \
                      -crl_check_all -untrusted "$CERT_CHAIN_FILE" "$CERT_CHAIN_FILE" &> "$OUT_STREAM"; then
    echo "TSA certificate from $TSA_URL could not be validated."
    return 1
  fi
  #validate token
  if ! openssl ts -verify -digest "$DIGEST" -in "$TOKEN_FILE" -token_in -attime "$TOKEN_UNIXTIME" \
                  -CApath "$CA_PATH" -untrusted "$CERT_CHAIN_FILE" 2> "$OUT_STREAM"; then
    echo "Token from $TSA_URL could not be validated."
    return 1
  fi
}