What is the algorithm identifier when using AES in CBC mode with ISO 10126 padding inside of PKCS#7?

Question

I want to use PKCS#7 as a container format for some encrypted, signed content and we need to use AES in CBC mode with ISO 10126 based padding. I can't seem to find a concrete reference to an algorithm identifier to use for this combination. I can invent my own but would then lose interoperability with other tools.

Answer 1

The padding mode does not need to be specified. This is true because PKCS#7 (like many other cryptographic containers) uses ASN.1/BER encoding. As the BER encoding contains the exact length of the message inside the padded plain text, the actual value of the padding doesn't matter.

Answer 2

It turns out that the padding mode does not need to be specified, only the crypto algorithm matters.

Answer 3

I believe if you are using PKCS#7 as your format, the padding is dictated to be, well, "PKCS7" padding (n octets of value 'n', see RFC 2315, section 10.3.2).

The algorithm identifier(s) for AES/CBC are given in e.g. RFC 3565:

-- AES information object identifiers --

aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840)
               organization(1) gov(101) csor(3)_ nistAlgorithms(4)  1 }

-- AES using CBC-chaining mode for key sizes of 128, 192, 256

id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 }
id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 }
id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }

So, e.g. the OID for 128-bit AES in CBC mode is 2.16.840.1.101.3.4.1.2

Technically, these are for CMS, which is essentially the updated PKCS#7.