How to find out symmetric encryption algorithm used in PGP encryption (with GPG)?

Question

I would like to be able to check which (inner) symmetric algorithm was used to encrypt a file with GPG using a public key.

Somewhere I've read this can be done with --list-packets, so I tried

$ gpg --list-packets encrypted_file.asc 
gpg: encrypted with 256-Bit ECDH key, ID 0865135E90D1AF38, created 2020-03-20
      "First Last <[email protected]>"
# off=0 ctb=84 tag=1 hlen=2 plen=94
:pubkey enc packet: version 3, algo 18, keyid 0865135E90D1AF38
    data: [263 bits]
    data: [392 bits]
# off=96 ctb=d2 tag=18 hlen=3 plen=1965 new-ctb
:encrypted data packet:
    length: 1965
    mdc_method: 2
# off=118 ctb=a3 tag=8 hlen=1 plen=0 indeterminate
:compressed packet: algo=1
# off=120 ctb=cb tag=11 hlen=2 plen=0 partial new-ctb
:literal data packet:
    mode b (62), created 0, name="_CONSOLE",
    raw data: unknown length

I thought the symmetric encryption algorithm was encoded in mdc_method, but whatever algorithm I use to encrypt, it always displays 2 there. So this doesn't seem to be the correct approach.

How can I find out the symmetric algorithm?

Answer 1

The trick is to add option -vv --show-session-key to the command:

$ gpg -vv --show-session-key --list-packets encrypted_file.asc 

This adds a line like the following to the output:

gpg: AES192 encrypted data
gpg: session key: '8:32050C047C47C519E76901EFC47FDFED0CD87CDB85809AFE'

The number before the colon (8 here) is the symmetric algorithm used to encrypt the file contents according to this list (taken from GnuPG sources):

typedef enum
 {
 CIPHER_ALGO_NONE = 0,
 CIPHER_ALGO_IDEA = 1,
 CIPHER_ALGO_3DES = 2,
 CIPHER_ALGO_CAST5 = 3,
 CIPHER_ALGO_BLOWFISH = 4, /* 128 bit */
 /* 5 & 6 are reserved */
 CIPHER_ALGO_AES = 7,
 CIPHER_ALGO_AES192 = 8,
 CIPHER_ALGO_AES256 = 9,
 CIPHER_ALGO_TWOFISH = 10, /* 256 bit */
 CIPHER_ALGO_CAMELLIA128 = 11,
 CIPHER_ALGO_CAMELLIA192 = 12,
 CIPHER_ALGO_CAMELLIA256 = 13
 }
cipher_algo_t;

So in this case, the file was encoded using AES-192.

Note that AES means AES-128.