Decode a base64 string and encode it as hex using xxd

Question

First, I take a base64 encoded string and decode it:

local base64_str="OQbb8rXnj/DwvglW018uP/1tqldwiJMbjxBhX7ZqwTw="
echo "${base64_str}" | base64 --decode > foo.txt

The size of the binary file is 32 bytes based on: wc -c < foo.txt

I use xxd to encode the value in the file to hex format:

xxd -p ./foo.txt ./foo.hex.txt

The hex value in the file foo.hex.txt is:

3906dbf2b5e78ff0f0be0956d35f2e3ffd6daa577088931b8f10615fb66a
c13c

The size of the encoded hash file is 66 bytes using wc -c < foo.hex.txt

I would like to take the base64 string and convert it to hex such that it remains a 32 byte string that I can use with openssl to encrypt and decrypt using aes-256 ciphers.

local iv_hex=$(base64_to_hex "${iv}")
local key_hex=$(base64_to_hex "${key}")

openssl enc -aes-256-ctr -K "${key_hex}" -iv "${iv_hex}" -in "${input_file}" -out "${output_file}"

look at the -c <number> option for xxd; with a large enough value (eg, in this case -c 64) the xxd output shows up on a single line; you can also go larger (eg, -c 100) without negative issues (ie, it'll still print the 64 characters on a single line); someone else asked a similar/related question a couple days ago; similarly for the iv_hex key, use -c 32 — markp-fuso
– markp-fuso, Commented Apr 10 at 13:10

F. Hauri - Give Up GitHub · Accepted Answer · 2025-04-12 12:31:09Z

Convert small base64 encoded string to hexadecimal

1. Reducing forks

Accepted answer do offer a solution with a lot of forks! I hate useless forks!

There is my short base64 to hexadecimal converter:

b64toHex() {
    local _arLines
    mapfile -t _arLines < <(base64 -d <<< "$2" | xxd -p)
    printf -v "${1:-hexString}" %s "${_arLines[@]}"
}

In order to avoid fork like myVar=$(myFunc args), this function will only populate a variable and won't print out anything.

b64toHex myVar "OQbb8rXnj/DwvglW018uP/1tqldwiJMbjxBhX7ZqwTw="
echo $myVar

3906dbf2b5e78ff0f0be0956d35f2e3ffd6daa577088931b8f10615fb66ac13c

Then you could use:

b64toHex iv_hex "${iv}"
b64toHex key_hex "${key}"

See at bottom of this for execution time comparison.

2. Pure bash way:

This don't depend on xxd or base64 to be installed. (And without forks, will be significantly quicker than running 3 forks! Keep in mind, if you plan to run this repetitively!)

From this: Bash script - decode encoded string to byte array, on my website: base64decoder.sh.txt, base64decoder.sh, with a very small modification**:

**At line 41:
- ((_ar==0)) && printf -v _res %b "${_res[@]/#/\\x}"
+ ((_ar==0)) && printf -v _res %s "${_res[@]}"

First prepare a read-only array as:

declare -a B64=( {A..Z} {a..z} {0..9} + / '=' )
declare -Ai 'B64R=()'
for i in "${!B64[@]}"; do B64R["${B64[i]}"]=i%64; done
declare -r B64R
unset B64

Then b64ToHex function:

b64ToHex() {
    local _4B _Tail _hVal _v _opt OPTIND
    local -i iFd _24b _ar
    while getopts "av:" _opt; do case $_opt in
        a) _ar=1;; v) _v=${OPTARG};; *) return 1;; esac; done
    shift $((OPTIND-1))
    if [[ $_v ]];then local -n _res=${_v}; else local _res; fi
    if [[ $1 ]]; then    exec {iFd}<<<"$1"          # Open Input FD from string
    else                 exec {iFd}<&0        ; fi  # Open Input FD from STDIN
    _res=()
    while read -rn4 -u $iFd _4B; do
        if [[ "$_4B" ]]; then
            _Tail=$_4B
            _24b=" B64R['${_4B::1}'] << 18 | B64R['${_4B:1:1}'] << 12 |
                   B64R['${_4B:2:1}'] << 6 | B64R['${_4B:3:1}'] "
            printf -v _hval %02x\  $((_24b>>16)) $((_24b>>8&255)) $((_24b&255))
            read -ra _hval <<<"$_hval"
            _res+=("${_hval[@]}")
        fi
    done
    exec {iFd}<&-
    _Tail=${_Tail##*([^=])}
    while [[ $_Tail ]]; do
        unset "_res[-1]"
        _Tail=${_Tail:1}
    done
    ((_ar==0)) && printf -v _res %s "${_res[@]}" && _res=("${_res[0]}")
    [[ -z $_v ]] && echo "${_res[@]}"
}

If bash loop are known to be slow, doing a loop over only 32 byte will be significantly quicker and less system expansive than running four forks!

Usage from STDIN:

b64ToHex <<<"OQbb8rXnj/DwvglW018uP/1tqldwiJMbjxBhX7ZqwTw="

3906dbf2b5e78ff0f0be0956d35f2e3ffd6daa577088931b8f10615fb66ac13c

From an argument:

b64ToHex "OQbb8rXnj/DwvglW018uP/1tqldwiJMbjxBhX7ZqwTw="

3906dbf2b5e78ff0f0be0956d35f2e3ffd6daa577088931b8f10615fb66ac13c

Assign a variable:

b64ToHex -v someVar "OQbb8rXnj/DwvglW018uP/1tqldwiJMbjxBhX7ZqwTw="
echo "$someVar"

3906dbf2b5e78ff0f0be0956d35f2e3ffd6daa577088931b8f10615fb66ac13c

Then from, to variables:

b64ToHex -v iv_hex "${iv}"
b64ToHex -v key_hex "${key}"

Note: this is done without any fork.

For fun: retrieving original base64 string, with bash V5.1+, you could:

shopt -s extglob
printf %b ${someVar//??/\\x& } | base64

OQbb8rXnj/DwvglW018uP/1tqldwiJMbjxBhX7ZqwTw=

3. Pure bash way, but usign bash V5.2+

Same script, but by using patsub_replacement and mapfile, I could do this without any bash loop!

declare -a B64=( {A..Z} {a..z} {0..9} + / '=' )
printf -v _b64_tstr '["\44{B64[%d]}"]=%%d%%%%64 ' {0..64}
# shellcheck disable=SC2059  # format is variable.
printf -v _b64_tstr "$_b64_tstr" {0..64}
declare -Ai "B64R=($_b64_tstr)"
unset B64 _b64_tstr
declare -r B64R

b64ToHex52() {
    local _line _Tail _v _opt OPTIND _resArry
    local -i iFd _ar
    while getopts "av:" _opt; do case $_opt in
        a) _ar=1;; v) _v=${OPTARG};; *) return 1;; esac; done
    shift $((OPTIND-1))
    if [[ $_v ]];then local -n _res=${_v}; else local _res; fi
    if [[ $1 ]]; then    exec {iFd}<<<"$1"     # Open Input FD from string
    else                 exec {iFd}<&0   ; fi  # Open Input FD from STDIN
    mapfile -tu $iFd _lines
    read -ra _resArry <<<"${_lines[*]//?/& }"
    printf -v _tmpStr '"B64R[%s]<<18|B64R[%s]<<12|B64R[%s]<<6|B64R[%s]" ' \
           "${_resArry[@]}"
    local -ia "_tmpArry=($_tmpStr)"
    printf -v _tmpStr '%06x' "${_tmpArry[@]}"
    read -ra _res <<<"${_tmpStr//??/& }"
    exec {iFd}<&-
    _Tail=${_lines[-1]##*([^=])}
    _res=("${_res[@]::${#_res[@]}-${#_Tail}}")
    ((_ar==0)) && printf -v _res %s "${_res[@]}" && _res=("${_res[0]}")
    [[ -z $_v ]] && echo "${_res[@]}"
}

4. Execution time comparison

Well, now a little comparison test by doing repetitively same conversion to compute execution time.

I now have 4 functions:

b64toHex My version using base64 and xxd
b64ToHex My pure bash version (notice upper T)
b64ToHex52 My pure bash using bash version 5.2+
base64_to_hex from accepted answer.

Here's my little test function:

testB64decoders(){ 
    local TIMEFORMAT='r %3lR,  u %3lU,  s %3lS, p %P' bunch \
        inString="${2:-SGVsbG8gd29ybGQhIFRoaXMgaXMgYSB0ZXN0IHN0cmluZy4=}"
    printf -v bunch '%*s' ${1:-100} ''
    mapfile -t bunch <<<"${bunch// /$'\n'}"
    printf ' - %-29s: ' "3 fork (base64 | xxd)";
    time for i in "${bunch[@]}"; do b64toHex hx "$inString"; done
    printf ' - %-29s: ' "Pure bash";
    time for i in "${bunch[@]}"; do b64ToHex -v Hx "$inString"; done;
    printf ' - %-29s: ' "Pure bash V5.2+";
    time for i in "${bunch[@]}"; do b64ToHex52 -v Hx5 "$inString"; done;
    printf ' - %-29s: ' "5 fork =\$(echo| base64 | xxd)";
    time for i in "${bunch[@]}"; do hex=$(base64_to_hex "$inString"); done;
    [[ $hx == "$hex" ]] && [[ $Hx == "$hx" ]] && [[ $Hx5 == "$hx" ]] &&
        printf -v inString '%b' ${hx//??/\\x&} &&
        printf 'Hopefully result strings are same (%s).\n' "${inString@Q}"
}

Let's show with a small thousand of operation:

testB64decoders 1000

Could produce something like:

 - 3 fork (base64 | xxd)        : r 0m2.032s,  u 0m2.315s,  s 0m0.699s, p 148.32
 - Pure bash                    : r 0m0.965s,  u 0m0.735s,  s 0m0.213s, p 98.14
 - Pure bash V5.2+              : r 0m0.571s,  u 0m0.477s,  s 0m0.088s, p 98.91
 - 5 fork =$(echo| base64 | xxd): r 0m2.433s,  u 0m3.242s,  s 0m0.989s, p 173.92

Where r for real, u: user, s: system time and p: cpu percentage is: 100 * ( u + s ) / r

pure bash method is quicker,
pure bash method using bash 5.2+ is significantly quicker,
version using xxd and base64 with only two fork will even be a little quicker than
version using four forks (a subshell to run three more forks to xxd, base64 and tr). They are the slowest, and yes: two more forks do have system footprint.

Note: on a multicore system, user time is bigger than real time, thanks to parallelization

On my Raspberry-Pi II model B, I had to reduce my test down to 20 loops.

testB64decoders 20

Did produce on my RPi II:

 - 3 fork (base64 | xxd)        : r 0m2.134s,  u 0m0.289s,  s 0m0.704s, p 46.55
 - Pure bash                    : r 0m1.931s,  u 0m0.890s,  s 0m0.104s, p 51.49
 - Pure bash V5.2+              : r 0m0.874s,  u 0m0.354s,  s 0m0.078s, p 49.36
 - 5 fork =$(echo| base64 | xxd): r 0m3.399s,  u 0m0.787s,  s 0m0.847s, p 48.06
Hopefully result strings are same ('Hello world! This is a test string.').

bash loop are known to be slow, but a loop over only 32 byte will be quicker and less system expansive than running 3 forks!
I've edited my answer to offer a version using base64 | xxd, but quicker and less system expansive than accepted answer.

pynexj · Accepted Answer · 2025-04-10 12:48:51Z

3

xxd -p may have \n chars in the output so you need to remove them:

base64_to_hex()
{
    echo "$1" | base64 --decode | xxd -p | tr -d '\n'
}

Use your base64 string as example:

$ hex=$( base64_to_hex OQbb8rXnj/DwvglW018uP/1tqldwiJMbjxBhX7ZqwTw= )
$ echo $hex
3906dbf2b5e78ff0f0be0956d35f2e3ffd6daa577088931b8f10615fb66ac13c

answered Apr 10 at 12:48

pynexj

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1 Comment

F. Hauri - Give Up GitHub Apr 11 at 7:13

Avoid useless fork!! Use base64 --decode <<<"$1" and drop useless echo "$1" | ...

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Decode a base64 string and encode it as hex using xxd

2 Answers 2

Convert small base64 encoded string to hexadecimal

1. Reducing forks

2. Pure bash way:

Usage from STDIN:

From an argument:

Assign a variable:

3. Pure bash way, but usign bash V5.2+

4. Execution time comparison

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2 Answers 2

Convert small base64 encoded string to hexadecimal

1. Reducing forks

2. Pure bash way:

Usage from STDIN:

From an argument:

Assign a variable:

3. Pure bash way, but usign bash V5.2+

4. Execution time comparison

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