Add RsaPublicKey::new_with_max_size; 4096-bit default max (#176)

This constructor accepts a configurable maximum key size which can be used in applications that need to deal with unusually large RSA public keys, such as OpenPGP. With the ability to configure an upper limit, this makes it possible to lower the default maximum key size to 4096-bits, which is a reasonable upper limit for most applications. Fixes #166
RustCrypto · Aug 9, 2022 · e7f4810 · e7f4810
1 parent 8d3fe53
commit e7f4810
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Showing 2 changed files with 105 additions and 15 deletions.
diff --git a/src/errors.rs b/src/errors.rs
@@ -1,7 +1,7 @@
 pub type Result<T> = core::result::Result<T, Error>;
 
 /// Error types
-#[derive(Debug)]
+#[derive(Debug, Eq, PartialEq)]
 #[non_exhaustive]
 pub enum Error {
     InvalidPaddingScheme,

diff --git a/src/key.rs b/src/key.rs
@@ -16,10 +16,6 @@ use crate::padding::PaddingScheme;
 use crate::raw::{DecryptionPrimitive, EncryptionPrimitive};
 use crate::{oaep, pkcs1v15, pss};
 
-const MIN_PUB_EXPONENT: u64 = 2;
-const MAX_PUB_EXPONENT: u64 = (1 << 33) - 1;
-const MAX_MODULUS_BITS: usize = 16384;
-
 pub trait PublicKeyParts {
     /// Returns the modulus of the key.
     fn n(&self) -> &BigUint;
@@ -229,11 +225,27 @@ impl PublicKey for RsaPublicKey {
 }
 
 impl RsaPublicKey {
-    /// Create a new key from its components.
+    /// Minimum value of the public exponent `e`.
+    pub const MIN_PUB_EXPONENT: u64 = 2;
+
+    /// Maximum value of the public exponent `e`.
+    pub const MAX_PUB_EXPONENT: u64 = (1 << 33) - 1;
+
+    /// Maximum size of the modulus `n` in bits.
+    pub const MAX_SIZE: usize = 4096;
+
+    /// Create a new public key from its components.
+    ///
+    /// This function accepts public keys with a modulus size up to 4096-bits,
+    /// i.e. [`RsaPublicKey::MAX_SIZE`].
     pub fn new(n: BigUint, e: BigUint) -> Result<Self> {
-        let k = RsaPublicKey { n, e };
-        check_public(&k)?;
+        Self::new_with_max_size(n, e, Self::MAX_SIZE)
+    }
 
+    /// Create a new public key from its components.
+    pub fn new_with_max_size(n: BigUint, e: BigUint, max_size: usize) -> Result<Self> {
+        let k = RsaPublicKey { n, e };
+        check_public_with_max_size(&k, max_size)?;
         Ok(k)
     }
 }
@@ -336,10 +348,9 @@ impl RsaPrivateKey {
     /// Get the public key from the private key, cloning `n` and `e`.
     ///
     /// Generally this is not needed since `RsaPrivateKey` implements the `PublicKey` trait,
-    /// but it can occationally be useful to discard the private information entirely.
+    /// but it can occasionally be useful to discard the private information entirely.
     pub fn to_public_key(&self) -> RsaPublicKey {
-        // Safe to unwrap since n and e are already verified.
-        RsaPublicKey::new(self.n().clone(), self.e().clone()).unwrap()
+        self.pubkey_components.clone()
     }
 
     /// Performs some calculations to speed up private key operations.
@@ -409,7 +420,7 @@ impl RsaPrivateKey {
     }
 
     /// Performs basic sanity checks on the key.
-    /// Returns `Ok(())` if everything is good, otherwise an approriate error.
+    /// Returns `Ok(())` if everything is good, otherwise an appropriate error.
     pub fn validate(&self) -> Result<()> {
         check_public(self)?;
 
@@ -549,7 +560,13 @@ impl RsaPrivateKey {
 /// Check that the public key is well formed and has an exponent within acceptable bounds.
 #[inline]
 pub fn check_public(public_key: &impl PublicKeyParts) -> Result<()> {
-    if public_key.n().bits() > MAX_MODULUS_BITS {
+    check_public_with_max_size(public_key, RsaPublicKey::MAX_SIZE)
+}
+
+/// Check that the public key is well formed and has an exponent within acceptable bounds.
+#[inline]
+fn check_public_with_max_size(public_key: &impl PublicKeyParts, max_size: usize) -> Result<()> {
+    if public_key.n().bits() > max_size {
         return Err(Error::ModulusTooLarge);
     }
 
@@ -558,11 +575,11 @@ pub fn check_public(public_key: &impl PublicKeyParts) -> Result<()> {
         .to_u64()
         .ok_or(Error::PublicExponentTooLarge)?;
 
-    if e < MIN_PUB_EXPONENT {
+    if e < RsaPublicKey::MIN_PUB_EXPONENT {
         return Err(Error::PublicExponentTooSmall);
     }
 
-    if e > MAX_PUB_EXPONENT {
+    if e > RsaPublicKey::MAX_PUB_EXPONENT {
         return Err(Error::PublicExponentTooLarge);
     }
 
@@ -601,6 +618,7 @@ mod tests {
 
     use alloc::string::String;
     use digest::{Digest, DynDigest};
+    use hex_literal::hex;
     use num_traits::{FromPrimitive, ToPrimitive};
     use rand_chacha::{
         rand_core::{RngCore, SeedableRng},
@@ -801,6 +819,78 @@ mod tests {
         .unwrap();
     }
 
+    #[test]
+    fn reject_oversized_private_key() {
+        // -----BEGIN PUBLIC KEY-----
+        // MIIEIjANBgkqhkiG9w0BAQEFAAOCBA8AMIIECgKCBAEAkMBiB8qsNVXAsJR6Xoto
+        // H1r2rtZl/xzUK2tIfy99aPE489u+5tLxCQhQf+a89158vSDpr2/xwgK8w9u0Xpu2
+        // m7XRKjVMS0Y6UIINFoeTc87rVXT92Scr47kNVcGmSFXez4BSDpS+LKpWwXN+0AQu
+        // +cmcfdtsx2862iEbqQvq4PwKGQJOdOR0yldH8O4yeJK/buvIOXRHjb++vtQND/xi
+        // bFGAcd9WJqvaOG7tclhbZ277mbO6ER+y9Lj7AyO8ywybWqNeHaVPHMysPhT7HUWI
+        // 17m59i1OpuVwwEnvzDQQEUf9d5hUmkLYb5qQzuf6Ddnx/04QJCKAgkhyr9CXgnV6
+        // vEZ3PKtpicCHRxk7eqTEmgBlgwqH5vflRFV1iywQMXJnuRhzWOQaXl/vb8v4HIvF
+        // 4TatEZKqfzpbyScLIiYbPEAhHXKdZMd2zY8hkSbicifePApAZmuNpAxxJDZzphh7
+        // r4lD6t8MPT/RUAdtrZfihqaBhduFI6YeVIy6emg05M6YWvlUyer7nYGaPRS1JqD4
+        // 0v7xOtme5I8Qw6APiFPXhTqBK3occr7TgGb3V3lpC8Eq+esNHrji98R1fITkFXJW
+        // KdFcTWjBghPxiobUzMCFUrPIDJcWXeBzrARAryU+hXjEiFfzluXrps0B7RJQ/rLD
+        // LXeTn4vovUeHQVHa7YfoyWMy9pfqeVC+56LBK7SEIAvL0I3lrq5vIv+ZIuOAdbVg
+        // JiRy8DneCOk2LP3RnA8M0HSevYW93DiC+4h/l4ntjjiOfi6yRVOZ8WbVyXZ/83j4
+        // 6+pGWgvi0uMyb+btgOXjBQv7bGqdyHMc5Lqk5bF7ExETx51vKQMYCV4351caS6aX
+        // q16lYZATHgbTADEAZHdroDMJB+HMQaze9O6qU5ZO8wxxAjw89xry0dnoOQD/yA4H
+        // 7CRCo9vVDpV2hqIvHY9RI2T7cek28kmQpKvNvvK+ovmM138dHKViWULHk0fBRt7m
+        // 4wQ+tiL2PmJ/Tr8g1gVhM6S9D1XdE9z0KeDnODCWn1Q8sx2G2ah4ynnYQURDWcwO
+        // McAoP6bdJ7cCt+4F2tEsMPf4S/EwlnjvuNoQjvztxCPahYe9EnyggtQXyHJveIn7
+        // gDJsP6b93VB6x4QbLy5ch4DUhqDWginuKVeo7CTgDkq03j/IEaS1BHwreSDQceny
+        // +bYWONwV+4TMpGytKOHvU5288kmHbyZHdXuaXk8LLqbnqr30fa6Cbp4llCi9sH5a
+        // Kmi5jxQfVTe+elkMs7oVsLsVgkZS6NqPcOuEckAFijNqG223+IJoqvifCzO5Bdcs
+        // JTOLE+YaUYc8LUJwIaPykgcXmtMvQjeT8MCQ3aAlzkHfDpSvvICrXtqbGiaKolU6
+        // mQIDAQAB
+        // -----END PUBLIC KEY-----
+
+        let n = BigUint::from_bytes_be(&hex!(
+            "
+            90c06207caac3555c0b0947a5e8b681f5af6aed665ff1cd42b6b487f2f7d68f1
+            38f3dbbee6d2f10908507fe6bcf75e7cbd20e9af6ff1c202bcc3dbb45e9bb69b
+            b5d12a354c4b463a50820d16879373ceeb5574fdd9272be3b90d55c1a64855de
+            cf80520e94be2caa56c1737ed0042ef9c99c7ddb6cc76f3ada211ba90beae0fc
+            0a19024e74e474ca5747f0ee327892bf6eebc83974478dbfbebed40d0ffc626c
+            518071df5626abda386eed72585b676efb99b3ba111fb2f4b8fb0323bccb0c9b
+            5aa35e1da54f1cccac3e14fb1d4588d7b9b9f62d4ea6e570c049efcc34101147
+            fd7798549a42d86f9a90cee7fa0dd9f1ff4e10242280824872afd09782757abc
+            46773cab6989c08747193b7aa4c49a0065830a87e6f7e54455758b2c10317267
+            b9187358e41a5e5fef6fcbf81c8bc5e136ad1192aa7f3a5bc9270b22261b3c40
+            211d729d64c776cd8f219126e27227de3c0a40666b8da40c71243673a6187baf
+            8943eadf0c3d3fd150076dad97e286a68185db8523a61e548cba7a6834e4ce98
+            5af954c9eafb9d819a3d14b526a0f8d2fef13ad99ee48f10c3a00f8853d7853a
+            812b7a1c72bed38066f75779690bc12af9eb0d1eb8e2f7c4757c84e415725629
+            d15c4d68c18213f18a86d4ccc08552b3c80c97165de073ac0440af253e8578c4
+            8857f396e5eba6cd01ed1250feb2c32d77939f8be8bd47874151daed87e8c963
+            32f697ea7950bee7a2c12bb484200bcbd08de5aeae6f22ff9922e38075b56026
+            2472f039de08e9362cfdd19c0f0cd0749ebd85bddc3882fb887f9789ed8e388e
+            7e2eb2455399f166d5c9767ff378f8ebea465a0be2d2e3326fe6ed80e5e3050b
+            fb6c6a9dc8731ce4baa4e5b17b131113c79d6f290318095e37e7571a4ba697ab
+            5ea56190131e06d300310064776ba0330907e1cc41acdef4eeaa53964ef30c71
+            023c3cf71af2d1d9e83900ffc80e07ec2442a3dbd50e957686a22f1d8f512364
+            fb71e936f24990a4abcdbef2bea2f98cd77f1d1ca5625942c79347c146dee6e3
+            043eb622f63e627f4ebf20d6056133a4bd0f55dd13dcf429e0e73830969f543c
+            b31d86d9a878ca79d841444359cc0e31c0283fa6dd27b702b7ee05dad12c30f7
+            f84bf1309678efb8da108efcedc423da8587bd127ca082d417c8726f7889fb80
+            326c3fa6fddd507ac7841b2f2e5c8780d486a0d68229ee2957a8ec24e00e4ab4
+            de3fc811a4b5047c2b7920d071e9f2f9b61638dc15fb84cca46cad28e1ef539d
+            bcf249876f2647757b9a5e4f0b2ea6e7aabdf47dae826e9e259428bdb07e5a2a
+            68b98f141f5537be7a590cb3ba15b0bb15824652e8da8f70eb847240058a336a
+            1b6db7f88268aaf89f0b33b905d72c25338b13e61a51873c2d427021a3f29207
+            179ad32f423793f0c090dda025ce41df0e94afbc80ab5eda9b1a268aa2553a99"
+        ));
+
+        let e = BigUint::from_u64(65537).unwrap();
+
+        assert_eq!(
+            RsaPublicKey::new(n, e).err().unwrap(),
+            Error::ModulusTooLarge
+        );
+    }
+
     fn get_private_key() -> RsaPrivateKey {
         // -----BEGIN RSA PRIVATE KEY-----
         // MIIEpAIBAAKCAQEA05e4TZikwmE47RtpWoEG6tkdVTvwYEG2LT/cUKBB4iK49FKW