IV in RSA Encrypted Private Key

Question

I have been provided an RSA encrypted private key which was generated in NodeJS (crypto) using the following:

generateKeyPairSync( "rsa", {
        modulusLength: 4096,
        publicKeyEncoding: {
          type: "spki",
          format: "pem"
        },
        privateKeyEncoding: {
          type: "pkcs8",
          format: "pem",
          cipher: "aes-256-cbc",
          passphrase: "aWCTJPET9fL7UBTp97hX99gdofeWKUf5tuxSuJeST2sEkyvkyinrfrj6EiSUTErF"
        }
      } )

Note: enforcing padding in encrypt and decrypt padding: RSA_PKCS1_PADDING

My task is to get the PRIVATE KEY by decrypting the ENCRYPTED PRIVATE KEY. I have the password as shown above and the other particulars. Everything but the IV.

This will have to be done manually as I have to use Java and it has a few issues:

java.security.NoSuchAlgorithmException: Cannot find any provider supporting 1.2.840.113549.1.5.13  (Password-Based Encryption Scheme 2 (PBES2))

Unfortunately, the library used within NodeJS (crypto) to generate the ENCRYPTED PRIVATE KEY does not supply the IV which is required in combination with the passphrase to decrypt the AES-256 CBC encrypted PRIVATE KEY.

I am reading that the IV can be acquired from the key and this is how the NodeJS library is able to decrypt the file while only asking for the passphrase from the user.

I would like to know how I can derive the IV from the ENCRYPTED PRIVATE KEY.

An IV in AES is always the size of the block, which in AES's case is 128-bits, or 16 bytes. So the way I am looking at it is which 16 bytes are the IV in the encrypted private key. It would also be nice to know how from a 16byte [] to reconstruct the bytes to get the IV.

I attach all forms of the key--encrypted, decrypted and the associated public key--as you may need it for testing.

ENCRYPTED PRIVATE KEY

DECRYPTED PRIVATE KEY

PUBLIC KEY

Actually this OP doesn't need it, see stackoverflow.com/questions/66286457/… -- but someone else in a different situation (not Java) might — dave_thompson_085, Feb 21, 2021 at 4:47

Swashbuckler · Accepted Answer · 2021-02-20 23:51:35Z

Run

openssl asn1parse -in keyfileName

You'll see a bunch of output, I believe the field after "aes-256-cbc" is the IV

 0:d=0  hl=4 l=2477 cons: SEQUENCE
 4:d=1  hl=2 l=  87 cons: SEQUENCE
 6:d=2  hl=2 l=   9 prim: OBJECT            :PBES2
17:d=2  hl=2 l=  74 cons: SEQUENCE
19:d=3  hl=2 l=  41 cons: SEQUENCE
21:d=4  hl=2 l=   9 prim: OBJECT            :PBKDF2
32:d=4  hl=2 l=  28 cons: SEQUENCE
34:d=5  hl=2 l=   8 prim: OCTET STRING      [HEX DUMP]:459A7208041047CA
44:d=5  hl=2 l=   2 prim: INTEGER           :0800
48:d=5  hl=2 l=  12 cons: SEQUENCE
50:d=6  hl=2 l=   8 prim: OBJECT            :hmacWithSHA256
60:d=6  hl=2 l=   0 prim: NULL
62:d=3  hl=2 l=  29 cons: SEQUENCE
64:d=4  hl=2 l=   9 prim: OBJECT            :aes-256-cbc
75:d=4  hl=2 l=  16 prim: OCTET STRING      [HEXDUMP]:BC87885E3BC691E4D689AB9A3CF66C81

I believe the 0800 (i.e. 1024) is the number of iterations of PBKDF2 that's used.

It is; and the value just before 0800 is the salt. Adding -i (lower eye) to asn1parse makes the structure clearer. — dave_thompson_085, Feb 21, 2021 at 4:45
This might help others as well: stackoverflow.com/questions/66286457/… — DevilCode, Feb 24, 2021 at 17:17

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