CVE-2021-41117

keypair is a a RSA PEM key generator written in javascript. keypair implements a lot of cryptographic primitives on its own or by borrowing from other libraries where possible, including node-forge. An issue was discovered where this library was generating identical RSA keys used in SSH. This would mean that the library is generating identical P, Q (and thus N) values which, in practical terms, is impossible with RSA-2048 keys. Generating identical values, repeatedly, usually indicates an issue with poor random number generation, or, poor handling of CSPRNG output. Issue 1: Poor random number generation (GHSL-2021-1012). The library does not rely entirely on a platform provided CSPRNG, rather, it uses it's own counter-based CMAC approach. Where things go wrong is seeding the CMAC implementation with "true" random data in the function defaultSeedFile. In order to seed the AES-CMAC generator, the library will take two different approaches depending on the JavaScript execution environment. In a browser, the library will use window.crypto.getRandomValues(). However, in a nodeJS execution environment, the window object is not defined, so it goes down a much less secure solution, also of which has a bug in it. It does look like the library tries to use node's CSPRNG when possible unfortunately, it looks like the crypto object is null because a variable was declared with the same name, and set to null. So the node CSPRNG path is never taken. However, when window.crypto.getRandomValues() is not available, a Lehmer LCG random number generator is used to seed the CMAC counter, and the LCG is seeded with Math.random. While this is poor and would likely qualify in a security bug in itself, it does not explain the extreme frequency in which duplicate keys occur. The main flaw: The output from the Lehmer LCG is encoded incorrectly. The specific [line][https://github.com/juliangruber/keypair/blob/87c62f255baa12c1ec4f98a91600f82af80be6db/index.js#L1008] with the flaw is: b.putByte(String.fromCharCode(next & 0xFF)) The definition of putByte is util.ByteBuffer.prototype.putByte = function(b) {this.data += String.fromCharCode(b);};. Simplified, this is String.fromCharCode(String.fromCharCode(next & 0xFF)). The double String.fromCharCode is almost certainly unintentional and the source of weak seeding. Unfortunately, this does not result in an error. Rather, it results most of the buffer containing zeros. Since we are masking with 0xFF, we can determine that 97% of the output from the LCG are converted to zeros. The only outputs that result in meaningful values are outputs 48 through 57, inclusive. The impact is that each byte in the RNG seed has a 97% chance of being 0 due to incorrect conversion. When it is not, the bytes are 0 through 9. In summary, there are three immediate concerns: 1. The library has an insecure random number fallback path. Ideally the library would require a strong CSPRNG instead of attempting to use a LCG and Math.random. 2. The library does not correctly use a strong random number generator when run in NodeJS, even though a strong CSPRNG is available. 3. The fallback path has an issue in the implementation where a majority of the seed data is going to effectively be zero. Due to the poor random number generation, keypair generates RSA keys that are relatively easy to guess. This could enable an attacker to decrypt confidential messages or gain authorized access to an account belonging to the victim.

keypair is a a RSA PEM key generator written in javascript. keypair implements a lot of cryptographic primitives on its own or by borrowing from other libraries where possible, including node-forge. An issue was discovered where this library was generating identical RSA keys used in SSH. This would mean that the library is generating identical P, Q (and thus N) values which, in practical terms, is impossible with RSA-2048 keys. Generating identical values, repeatedly, usually indicates an issue with poor random number generation, or, poor handling of CSPRNG output. Issue 1: Poor random number generation (GHSL-2021-1012). The library does not rely entirely on a platform provided CSPRNG, rather, it uses it's own counter-based CMAC approach. Where things go wrong is seeding the CMAC implementation with "true" random data in the function defaultSeedFile. In order to seed the AES-CMAC generator, the library will take two different approaches depending on the JavaScript execution environment. In a browser, the library will use window.crypto.getRandomValues(). However, in a nodeJS execution environment, the window object is not defined, so it goes down a much less secure solution, also of which has a bug in it. It does look like the library tries to use node's CSPRNG when possible unfortunately, it looks like the crypto object is null because a variable was declared with the same name, and set to null. So the node CSPRNG path is never taken. However, when window.crypto.getRandomValues() is not available, a Lehmer LCG random number generator is used to seed the CMAC counter, and the LCG is seeded with Math.random. While this is poor and would likely qualify in a security bug in itself, it does not explain the extreme frequency in which duplicate keys occur. The main flaw: The output from the Lehmer LCG is encoded incorrectly. The specific [line][https://github.com/juliangruber/keypair/blob/87c62f255baa12c1ec4f98a91600f82af80be6db/index.js#L1008] with the flaw is: b.putByte(String.fromCharCode(next & 0xFF)) The definition of putByte is util.ByteBuffer.prototype.putByte = function(b) {this.data += String.fromCharCode(b);};. Simplified, this is String.fromCharCode(String.fromCharCode(next & 0xFF)). The double String.fromCharCode is almost certainly unintentional and the source of weak seeding. Unfortunately, this does not result in an error. Rather, it results most of the buffer containing zeros. Since we are masking with 0xFF, we can determine that 97% of the output from the LCG are converted to zeros. The only outputs that result in meaningful values are outputs 48 through 57, inclusive. The impact is that each byte in the RNG seed has a 97% chance of being 0 due to incorrect conversion. When it is not, the bytes are 0 through 9. In summary, there are three immediate concerns: 1. The library has an insecure random number fallback path. Ideally the library would require a strong CSPRNG instead of attempting to use a LCG and Math.random. 2. The library does not correctly use a strong random number generator when run in NodeJS, even though a strong CSPRNG is available. 3. The fallback path has an issue in the implementation where a majority of the seed data is going to effectively be zero. Due to the poor random number generation, keypair generates RSA keys that are relatively easy to guess. This could enable an attacker to decrypt confidential messages or gain authorized access to an account belonging to the victim.

Description and Impact

A bug in the pseudo-random number generator used by keypair versions up to and including 1.0.3 could allow for weak RSA key generation. This could enable an attacker to decrypt confidential messages or gain authorized access to an account belonging to the victim. We recommend replacing any RSA keys that were generated using keypair version 1.0.3 or earlier.

Fix

• The bug in the pseudo-random number generator is fixed in commit 9596418.
• If the crypto module is available, it is used instead of the pseudo-random number generator. Also fixed in 9596418

Additional Details

The specific line with the flaw is:

b.putByte(String.fromCharCode(next & 0xFF))

The definition of putByte is

util.ByteBuffer.prototype.putByte = function(b) {
  this.data += String.fromCharCode(b);
};

Simplified, this is String.fromCharCode(String.fromCharCode(next & 0xFF)). This results in most of the buffer containing zeros. An example generated buffer:

(Note: truncated for brevity)

\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x04\x00\x00\x00....\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00

Since it is masking with 0xFF, approximately 97% of the bytes are converted to zeros. The impact is that each byte in the RNG seed has a 97% chance of being 0 due to incorrect conversion.

Credit

This issue was reported to GitHub Security Lab by Ross Wheeler of Axosoft. It was discovered by Axosoft engineer Dan Suceava, who noticed that keypair was regularly generating duplicate RSA keys. GitHub security engineer @vcsjones (Kevin Jones) independently investigated the problem and identified the cause and source code location of the bug.

keypair is a a RSA PEM key generator written in javascript. keypair implements a lot of cryptographic primitives on its own or by borrowing from other libraries where possible, including node-forge. An issue was discovered where this library was generating identical RSA keys used in SSH. This would mean that the library is generating identical P, Q (and thus N) values which, in practical terms, is impossible with RSA-2048 keys. Generating identical values, repeatedly, usually indicates an issue with poor random number generation, or, poor handling of CSPRNG output. Issue 1: Poor random number generation (GHSL-2021-1012). The library does not rely entirely on a platform provided CSPRNG, rather, it uses it's own counter-based CMAC approach. Where things go wrong is seeding the CMAC implementation with "true" random data in the function defaultSeedFile. In order to seed the AES-CMAC generator, the library will take two different approaches depending on the JavaScript execution environment. In a browser, the library will use window.crypto.getRandomValues(). However, in a nodeJS execution environment, the window object is not defined, so it goes down a much less secure solution, also of which has a bug in it. It does look like the library tries to use node's CSPRNG when possible unfortunately, it looks like the crypto object is null because a variable was declared with the same name, and set to null. So the node CSPRNG path is never taken. However, when window.crypto.getRandomValues() is not available, a Lehmer LCG random number generator is used to seed the CMAC counter, and the LCG is seeded with Math.random. While this is poor and would likely qualify in a security bug in itself, it does not explain the extreme frequency in which duplicate keys occur. The main flaw: The output from the Lehmer LCG is encoded incorrectly. The specific [line][https://github.com/juliangruber/keypair/blob/87c62f255baa12c1ec4f98a91600f82af80be6db/index.js#L1008] with the flaw is: b.putByte(String.fromCharCode(next & 0xFF)) The definition of putByte is util.ByteBuffer.prototype.putByte = function(b) {this.data += String.fromCharCode(b);};. Simplified, this is String.fromCharCode(String.fromCharCode(next & 0xFF)). The double String.fromCharCode is almost certainly unintentional and the source of weak seeding. Unfortunately, this does not result in an error. Rather, it results most of the buffer containing zeros. Since we are masking with 0xFF, we can determine that 97% of the output from the LCG are converted to zeros. The only outputs that result in meaningful values are outputs 48 through 57, inclusive. The impact is that each byte in the RNG seed has a 97% chance of being 0 due to incorrect conversion. When it is not, the bytes are 0 through 9. In summary, there are three immediate concerns: 1. The library has an insecure random number fallback path. Ideally the library would require a strong CSPRNG instead of attempting to use a LCG and Math.random. 2. The library does not correctly use a strong random number generator when run in NodeJS, even though a strong CSPRNG is available. 3. The fallback path has an issue in the implementation where a majority of the seed data is going to effectively be zero. Due to the poor random number generation, keypair generates RSA keys that are relatively easy to guess. This could enable an attacker to decrypt confidential messages or gain authorized access to an account belonging to the victim.

keypair is a a RSA PEM key generator written in javascript. keypair implements a lot of cryptographic primitives on its own or by borrowing from other libraries where possible, including node-forge. An issue was discovered where this library was generating identical RSA keys used in SSH. This would mean that the library is generating identical P, Q (and thus N) values which, in practical terms, is impossible with RSA-2048 keys. Generating identical values, repeatedly, usually indicates an issue with poor random number generation, or, poor handling of CSPRNG output. Issue 1: Poor random number generation (GHSL-2021-1012). The library does not rely entirely on a platform provided CSPRNG, rather, it uses it's own counter-based CMAC approach. Where things go wrong is seeding the CMAC implementation with "true" random data in the function defaultSeedFile. In order to seed the AES-CMAC generator, the library will take two different approaches depending on the JavaScript execution environment. In a browser, the library will use window.crypto.getRandomValues(). However, in a nodeJS execution environment, the window object is not defined, so it goes down a much less secure solution, also of which has a bug in it. It does look like the library tries to use node's CSPRNG when possible unfortunately, it looks like the crypto object is null because a variable was declared with the same name, and set to null. So the node CSPRNG path is never taken. However, when window.crypto.getRandomValues() is not available, a Lehmer LCG random number generator is used to seed the CMAC counter, and the LCG is seeded with Math.random. While this is poor and would likely qualify in a security bug in itself, it does not explain the extreme frequency in which duplicate keys occur. The main flaw: The output from the Lehmer LCG is encoded incorrectly. The specific [line][https://github.com/juliangruber/keypair/blob/87c62f255baa12c1ec4f98a91600f82af80be6db/index.js#L1008] with the flaw is: b.putByte(String.fromCharCode(next & 0xFF)) The definition of putByte is util.ByteBuffer.prototype.putByte = function(b) {this.data += String.fromCharCode(b);};. Simplified, this is String.fromCharCode(String.fromCharCode(next & 0xFF)). The double String.fromCharCode is almost certainly unintentional and the source of weak seeding. Unfortunately, this does not result in an error. Rather, it results most of the buffer containing zeros. Since we are masking with 0xFF, we can determine that 97% of the output from the LCG are converted to zeros. The only outputs that result in meaningful values are outputs 48 through 57, inclusive. The impact is that each byte in the RNG seed has a 97% chance of being 0 due to incorrect conversion. When it is not, the bytes are 0 through 9. In summary, there are three immediate concerns: 1. The library has an insecure random number fallback path. Ideally the library would require a strong CSPRNG instead of attempting to use a LCG and Math.random. 2. The library does not correctly use a strong random number generator when run in NodeJS, even though a strong CSPRNG is available. 3. The fallback path has an issue in the implementation where a majority of the seed data is going to effectively be zero. Due to the poor random number generation, keypair generates RSA keys that are relatively easy to guess. This could enable an attacker to decrypt confidential messages or gain authorized access to an account belonging to the victim.

Description and Impact

A bug in the pseudo-random number generator used by keypair versions up to and including 1.0.3 could allow for weak RSA key generation. This could enable an attacker to decrypt confidential messages or gain authorized access to an account belonging to the victim. We recommend replacing any RSA keys that were generated using keypair version 1.0.3 or earlier.

Fix

• The bug in the pseudo-random number generator is fixed in commit 9596418.
• If the crypto module is available, it is used instead of the pseudo-random number generator. Also fixed in 9596418

Additional Details

The specific line with the flaw is:

b.putByte(String.fromCharCode(next & 0xFF))

The definition of putByte is

util.ByteBuffer.prototype.putByte = function(b) {
  this.data += String.fromCharCode(b);
};

Simplified, this is String.fromCharCode(String.fromCharCode(next & 0xFF)). This results in most of the buffer containing zeros. An example generated buffer:

(Note: truncated for brevity)

\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x04\x00\x00\x00....\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00

Since it is masking with 0xFF, approximately 97% of the bytes are converted to zeros. The impact is that each byte in the RNG seed has a 97% chance of being 0 due to incorrect conversion.

Credit

This issue was reported to GitHub Security Lab by Ross Wheeler of Axosoft. It was discovered by Axosoft engineer Dan Suceava, who noticed that keypair was regularly generating duplicate RSA keys. GitHub security engineer @vcsjones (Kevin Jones) independently investigated the problem and identified the cause and source code location of the bug.