ursa

ursa

This Node module provides wrappers for the RSA public/private key crypto functionality of OpenSSL.

This module was inspired by node-rsa by Chris Andrews. To be clear, there are a few lines that I (Danfuzz) used from its wscript build file, but other than that this code is new.

Building and Installing

npm install ursa

Or grab the source and

node-waf configure build

Testing

npm test

Or

node ./test/test.js

Usage

This library aims to be convenient to use, allowing one to pass in and get back regular string objects. However, it is also meant to be reasonably easy to use efficiently, allowing one to pass in and get back Buffer objects. Using Buffers is always the more efficient option.

All methods that can deal with strings take one or more arguments indicating the encoding to use when interpreting an argument or generating a result. These are limited to the usual encoding names that are valid for use with Buffers: base64 binary hex and utf8. If an encoding is left undefined and the argument is a string, then the encoding is always assumed to be utf8. If an argument is a Buffer, then the encoding (if defined at all) is ignored. An undefined output encoding is always interpreted as a request for a Buffer result.

The library knows how to read and output PEM format files for both public and private keys, and it can generate new private keys (aka keypairs).

The usual public-encryption / private-decryption operations are always done using padding mode RSA_PKCS1_OAEP_PADDING, which is the recommended mode for all new applications (as of this writing). Note that this mode builds-in a random element into every encryption operation, making it unnecessary to waste time or effort adding randomness in at a higher layer.

The less well-understood private-encryption / public-decryption operations (used for building signature mechanisms) are always done using padding mode RSA_PKCS1_PADDING. This doesn't build in any randomness (but that's not usually a problem for applications that use these operations).

See the doc comments and tests for the excruciating details, but here's a quick rundown of the available top-level exports and instance methods:

Top-Level Exports

ursa.createPrivateKey(pem, password, encoding)

Create and return a private key (aka a keypair) read in from the given PEM-format file. If defined, the given password is used to decrypt the PEM file.

The encoding, if specified, applies to both other arguments.

See "Public Key Methods" below for more details.

ursa.assertKey(obj)

Convenient shorthand for assert(ursa.isKey(obj)).

ursa.assertPrivateKey(obj)

Convenient shorthand for assert(ursa.isPrivateKey(obj)).

ursa.assertPublicKey(obj)

Convenient shorthand for assert(ursa.isPublicKey(obj)).

ursa.createPublicKey(pem, encoding)

Create and return a public key read in from the given PEM-format file. See "Public Key Methods" below for more details.

ursa.createSigner(algorithm)

Create and return a signer which can sign a hash generated with the named algorithm (such as "sha256" or "md5"). See "Signer Methods" below for more details.

This function is similar to crypto.createSign(), except this function takes a hash algorithm name (e.g., "sha256") and not a crypto+hash name combination (e.g., "RSA-SHA256").

ursa.createVerifier(algorithm)

Create and return a verifier which can verify a hash generated with the named algorithm (such as "sha256" or "md5"). See "Verifier Methods" below for more details.

This function is similar to crypto.createVerify(), except this function takes a hash algorithm name (e.g., "sha256") and not a crypto+hash name combination (e.g., "RSA-SHA256").

ursa.generatePrivateKey(modulusBits, exponent)

Create and return a freshly-generated private key (aka a keypair). The first argument indicates the number of bits in the modulus (1024 or more is generally considered secure). The second argument indicates the exponent value, which must be odd (65537 is the typical value; 3 and 17 are also common). Both arguments are optional and default to 2048 and 65537 (respectively).

Using the command-line openssl tool, this operation is equivalent to:

openssl genrsa -out key-name.pem <modulusBits>

for exponent 65537, or for exponent 3 with the additional option -3. (That tool doesn't support other exponents.)

ursa.isKey(obj)

Return true if the given object is a key object (public or private) that was created by this module. Return false if not.

ursa.isPrivateKey(obj)

Return true if the given object is a private key object that was created by this module. Return false if not.

ursa.isPublicKey(obj)

Return true if the given object is a public key object that was created by this module. Return false if not.

Note that, even though all the public key operations work on private keys, this function only returns true if the given object is a public key, per se.

ursa.sshFingerprint(sshKey, sshEncoding, outEncoding)

Return the SSH-style public key fingerprint of the given SSH-format public key (which was, perhaps, the result of a call to toPublicSsh() on a key object).

This is no more and no less than an MD5 hash of the given SSH-format public key. This function doesn't actually check to see if the given key is valid (garbage in, garbage out).

Using the command-line ssh-keygen tool, this operation is equivalent to:

ssh-keygen -l -f key-name.sshpub

This operation is also equivalent to this:

cat key-name.sshpub | awk '{print $2}' | base64 --decode | md5

Public Key Methods

These are all the methods available on public keys. These methods are also available on private keys (since private keys have all the underlying data necessary to perform the public-side operations).

encrypt(buf, bufEncoding, outEncoding)

This performs the "public encrypt" operation on the given buffer. The result is always a byte sequence that is the same size as the key associated with the instance. (For example, if the key is 2048 bits, then the result of this operation will be 2048 bits, aka 256 bytes.)

The input buffer is limited to be no larger than the key size minus 41 bytes.

This operation is always performed using padding mode RSA_PKCS1_OAEP_PADDING.

getExponent(encoding)

Get the public exponent as an unsigned big-endian byte sequence.

getModulus(encoding)

Get the public modulus as an unsigned big-endian byte sequence.

hashAndVerify(algorithm, buf, sig, encoding)

This is a friendly wrapper for verifying signatures. The given buffer is hashed using the named algorithm, and the result is verified against the given signature. This returns true if the hash and signature match and the signature was produced by the appropriate private key. This returns false if the signature is a valid signature (structurally) but doesn't match. This throws an exception in other cases.

The encoding, if specified, applies to both buffer-like arguments. The algorithm must always be a string.

publicDecrypt(buf, bufEncoding, outEncoding)

This performs the "public decrypt" operation on the given buffer. The result is always a byte sequence that is no more than the size of the key associated with the instance. (For example, if the key is 2048 bits, then the result of this operation will be no more than 2048 bits, aka 256 bytes.)

This operation is always performed using padding mode RSA_PKCS1_PADDING.

toPublicPem(encoding)

This converts the public key data into a PEM-format file.

toPublicSsh(encoding)

This converts the public key data into an SSH-format file. This is the file format one finds in SSH's authorized_keys and known_hosts files. When used in such files, the contents are base64-encoded and prefixed with the label ssh-rsa. Depending on context, the line a key appears on may also have a host name prefix (in known_hosts) or comment suffix (in authorized_keys).

Using the command-line ssh-keygen tool, this operation is equivalent to:

ssh-keygen -y -f key-name.pem > key-name.sshpub

toPublicSshFingerprint(encoding)

Return the SSH-style public key fingerprint of this key. See ursa.sshFingerprint(), above, for more details.

verify(algorithm, hash, sig, encoding)

This performs an RSA public-verify on the given hash buffer, which should be the result of performing the hash operation named by the algorithm (such as "sha256" or "md5") on some data. The signature buffer is checked to see if it contains a private-signed statement of the algorithm and hash. The method returns true if the signature and hash match, or false if the signature and hash don't match but the signature is at least a valid signature of some sort. In any other situation, this throws an exception.

The encoding, if specified, applies to both buffer-like arguments. The algorithm must always be a string.

This method is the underlying one used as part of the implementation of the higher-level and much friendlier ursa.createVerifier() and hashAndVerify().

ununseal(ununsealer)

This is an internal method that is used in the implementation of ursa.isKey() ursa.isPrivateKey() ursa.isPublicKey() and associated assertion functions. When called externally, it will always return undefined.

Private Key Methods

These are the methods available on private keys, above and beyond what is available for public keys.

decrypt(buf, bufEncoding, outEncoding)

This performs the "private decrypt" operation on the given buffer. The result is always a byte sequence that is no more than the size of the key associated with the instance. (For example, if the key is 2048 bits, then the result of this operation will be no more than 2048 bits, aka 256 bytes.)

This operation is always performed using padding mode RSA_PKCS1_OAEP_PADDING.

hashAndSign(algorithm, buf, bufEncoding, outEncoding)

This is a friendly wrapper for producing signatures. The given buffer is hashed using the named algorithm, and the result is signed using the private key held by this instance. The return value of this method is the signature.

privateEncrypt(buf, bufEncoding, outEncoding)

This performs the "private encrypt" operation on the given buffer. The result is always a byte sequence that is the same size as the key associated with the instance. (For example, if the key is 2048 bits, then the result of this operation will be 2048 bits, aka 256 bytes.)

The input buffer is limited to be no larger than the key size minus 12 bytes.

This operation is always performed using padding mode RSA_PKCS1_PADDING.

sign(algorithm, hash, hashEncoding, outEncoding)

This performs an RSA private-sign on the given hash buffer, which should be the result of performing the hash operation named by the algorithm (such as "sha256" or "md5") on some data. The result of this operation may later be passed to verify() on the corresponding public key.

This method is the underlying one used as part of the implementation of the higher-level and much friendlier ursa.createSigner() and hashAndSign().

toPrivatePem(encoding)

This converts the private key data into a PEM-format file. The result is not encrypted, so it behooves the user of this method to take care with the result if the key is sensitive from a security standpoint, which is often the case with such things. (YMMV of course.)

Signer Methods

These are the methods available on signer objects, which are returned by ursa.createSigner(). These are similar to the objects returned from crypto.createSign().

update(buf, bufEncoding)

Update the hash in-progress with the given data.

sign(privateKey, outEncoding)

Get the final hash of the data, and sign it using the private key. The return value is the signature, suitable for later verification.

Verifier Methods

These are the methods available on verifier objects, which are returned by ursa.createVerifier(). These are similar to the objects returned from crypto.createVerify().

update(buf, bufEncoding)

Update the hash in-progress with the given data.

verify(publicKey, sig, sigEncoding)

Get the final hash of the data, and verify that the given signature both matches it and was produced by the private key corresponding to the given public key.

This returns true if the signature and hash match appropriately, or false if the signature appears to be generally valid (e.g. structurally) yet doesn't match. This throws an exception in all other cases.

Contributing

Questions, comments, bug reports, and pull requests are all welcome. Submit them at the project on GitHub.

Bug reports that include steps-to-reproduce (including code) are the best. Even better, make them in the form of pull requests that update the test suite. Thanks!

Author

Dan Bornstein (personal website), supported by The Obvious Corporation.

License

Copyright 2012 The Obvious Corporation.

Licensed under the Apache License, Version 2.0. See the top-level file LICENSE.txt and (http://www.apache.org/licenses/LICENSE-2.0).