How to sign a document in python with M2Crypto using particular padding technique?

Question

I need to digitally sign some text in python using a private key stored in a .pem file. It seems like M2Crypto is the preferred way to do that these days, so that's what I'm using. I think I get most of it, but I'm confused about how to configure padding. To be specific, I need to verify the signature in an iPhone app, using a padding scheme called kSecPaddingPKCS1SHA1 and described like this:

Data to be signed is a SHA1 hash. Standard ASN.1 padding will be done, as well as PKCS1 padding of the underlying RSA operation.

Not being a crypto expert, I have only a fuzzy idea what this means. I've tried to look at some of the RFCs but found them impenetrable. I see that the encryption/decryption methods of RSA objects take padding types, but I don't see anything similar related to signature verification.

Any help, especially with code, will be appreciated.

(In some sense this is the converse of this question.)

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Ok, the answer given below is correct AFAICT. The following code generates a signature for text that validates on the iPhone using the kSecPaddingPKCS1SHA1 padding scheme.

from M2Crypto import EVP
privkey = EVP.load_key("privkey.pem")
privkey.sign_init()
privkey.sign_update(text)
signature = privkey.sign_final()

(Sorry to editorialize, but can I just say that crypto hackers are some of the lousiest documentation writers in the universe?)

Answer 1

AFAIK M2Crypto adds padding where it's required.

PKCS1 padding is the default.

But, (again only AFAIK), signatures don't have padding, padding is only added to encrypted data to prevent a possible attack.
EDIT: user caf, in a comment says that a padding is essnetial to a good signature. I'm still recommending you try it with the default M2Crypto behavior, it might add it.

On M2Crypto's generated docs you can see that the {public,private}_{encrypt,decrypt} methods have a padding option, which is PKCS1 by default, while the sign menthod has none.

IMO just give it a shot with the default M2Crypto params, it will probably work.