Merge #10657: Utils: Improvements to ECDSA key-handling code

63179d0 Scope the ECDSA constant sizes to CPubKey / CKey classes (Jack Grigg)
1ce9f0a Ensure that ECDSA constant sizes are correctly-sized (Jack Grigg)
48abe78 Remove redundant `= 0` initialisations (Jack Grigg)
17fa391 Specify ECDSA constant sizes as constants (Jack Grigg)
e4a1086 Update Debian copyright list (Jack Grigg)
e181dbe Add comments (Jack Grigg)
a3603ac Fix potential overflows in ECDSA DER parsers (Jack Grigg)

Pull request description:

  Mostly trivial, but includes fixes to potential overflows in the ECDSA DER parsers.

  Cherry-picked from Zcash PR https://github.com/zcash/zcash/pull/2335

Tree-SHA512: 8fcbd51b0bd6723e5d33fa5d592f7cb68ed182796a9b837ecc8217991ad69d6c970258617dc00eb378c8caa4cec5d6b304d9d2c066acd40cda98e4da68e0caa4

src/key.h

@@ -1,5 +1,6 @@
 // Copyright (c) 2009-2010 Satoshi Nakamoto
 // Copyright (c) 2009-2016 The Bitcoin Core developers
+// Copyright (c) 2017 The Zcash developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -15,25 +16,30 @@
 #include <vector>
 
 
-/**
- * secp256k1:
- * const unsigned int PRIVATE_KEY_SIZE = 279;
- * const unsigned int PUBLIC_KEY_SIZE  = 65;
- * const unsigned int SIGNATURE_SIZE   = 72;
- *
- * see www.keylength.com
- * script supports up to 75 for single byte push
- */
-
 /**
  * secure_allocator is defined in allocators.h
- * CPrivKey is a serialized private key, with all parameters included (279 bytes)
+ * CPrivKey is a serialized private key, with all parameters included
+ * (PRIVATE_KEY_SIZE bytes)
  */
 typedef std::vector<unsigned char, secure_allocator<unsigned char> > CPrivKey;
 
 /** An encapsulated private key. */
 class CKey
 {
+public:
+    /**
+     * secp256k1:
+     */
+    static const unsigned int PRIVATE_KEY_SIZE            = 279;
+    static const unsigned int COMPRESSED_PRIVATE_KEY_SIZE = 214;
+    /**
+     * see www.keylength.com
+     * script supports up to 75 for single byte push
+     */
+    static_assert(
+        PRIVATE_KEY_SIZE >= COMPRESSED_PRIVATE_KEY_SIZE,
+        "COMPRESSED_PRIVATE_KEY_SIZE is larger than PRIVATE_KEY_SIZE");
+
 private:
     //! Whether this private key is valid. We check for correctness when modifying the key
     //! data, so fValid should always correspond to the actual state.