extmod/modubinascii: Rewrite mod_binascii_a2b_base64.

This implementation ignores invalid characters in the input. This allows
it to decode the output of b2a_base64, and also mimics the behavior of
CPython.

Author: alex-robbins

extmod/modubinascii.c

@@ -105,54 +105,64 @@ mp_obj_t mod_binascii_unhexlify(mp_obj_t data) {
 }
 MP_DEFINE_CONST_FUN_OBJ_1(mod_binascii_unhexlify_obj, mod_binascii_unhexlify);
 
+// If ch is a character in the base64 alphabet, and is not a pad character, then
+// the corresponding integer between 0 and 63, inclusively, is returned.
+// Otherwise, -1 is returned.
+static int mod_binascii_sextet(byte ch) {
+    if (ch >= 'A' && ch <= 'Z') {
+        return ch - 'A';
+    } else if (ch >= 'a' && ch <= 'z') {
+        return ch - 'a' + 26;
+    } else if (ch >= '0' && ch <= '9') {
+        return ch - '0' + 52;
+    } else if (ch == '+') {
+        return 62;
+    } else if (ch == '/') {
+        return 63;
+    } else {
+        return -1;
+    }
+}
+
 mp_obj_t mod_binascii_a2b_base64(mp_obj_t data) {
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(data, &bufinfo, MP_BUFFER_READ);
-    if (bufinfo.len % 4 != 0) {
-        mp_raise_ValueError("incorrect padding");
-    }
+    byte *in = bufinfo.buf;
 
     vstr_t vstr;
-    byte *in = bufinfo.buf;
-    if (bufinfo.len == 0) {
-        vstr_init_len(&vstr, 0);
-    }
-    else {
-        vstr_init_len(&vstr, ((bufinfo.len / 4) * 3) - ((in[bufinfo.len-1] == '=') ? ((in[bufinfo.len-2] == '=') ? 2 : 1 ) : 0));
-    }
-    byte *out = (byte*)vstr.buf;
-    for (mp_uint_t i = bufinfo.len; i; i -= 4) {
-        char hold[4];
-        for (int j = 4; j--;) {
-            if (in[j] >= 'A' && in[j] <= 'Z') {
-                hold[j] = in[j] - 'A';
-            } else if (in[j] >= 'a' && in[j] <= 'z') {
-                hold[j] = in[j] - 'a' + 26;
-            } else if (in[j] >= '0' && in[j] <= '9') {
-                hold[j] = in[j] - '0' + 52;
-            } else if (in[j] == '+') {
-                hold[j] = 62;
-            } else if (in[j] == '/') {
-                hold[j] = 63;
-            } else if (in[j] == '=') {
-                if (j < 2 || i > 4) {
-                    mp_raise_ValueError("incorrect padding");
-                }
-                hold[j] = 64;
-            } else {
-                mp_raise_ValueError("invalid character");
+    vstr_init(&vstr, (bufinfo.len / 4) * 3 + 1); // Potentially over-allocate
+    byte *out = (byte *)vstr.buf;
+
+    uint shift = 0;
+    int nbits = 0; // Number of meaningful bits in shift
+    bool hadpad = false; // Had a pad character since last valid character
+    for (size_t i = 0; i < bufinfo.len; i++) {
+        if (in[i] == '=') {
+            if ((nbits == 2) || ((nbits == 4) && hadpad)) {
+                nbits = 0;
+                break;
             }
+            hadpad = true;
         }
-        in += 4;
 
-        *out++ = (hold[0]) << 2 | (hold[1]) >> 4;
-        if (hold[2] != 64) {
-            *out++ = (hold[1] & 0x0F) << 4 | hold[2] >> 2;
-            if (hold[3] != 64) {
-                *out++ = (hold[2] & 0x03) << 6 | hold[3];
-            }
+        int sextet = mod_binascii_sextet(in[i]);
+        if (sextet == -1) {
+            continue;
+        }
+        hadpad = false;
+        shift = (shift << 6) | sextet;
+        nbits += 6;
+
+        if (nbits >= 8) {
+            nbits -= 8;
+            out[vstr.len++] = (shift >> nbits) & 0xFF;
         }
     }
+
+    if (nbits) {
+        mp_raise_ValueError("incorrect padding");
+    }
+
     return mp_obj_new_str_from_vstr(&mp_type_bytes, &vstr);
 }
 MP_DEFINE_CONST_FUN_OBJ_1(mod_binascii_a2b_base64_obj, mod_binascii_a2b_base64);

tests/extmod/ubinascii_a2b_base64.py

@@ -21,6 +21,13 @@
 print(binascii.a2b_base64(b'f4D+')) # convert '+'
 print(binascii.a2b_base64(b'MTIzNEFCQ0RhYmNk'))
 
+# Ignore invalid characters and pad sequences
+print(binascii.a2b_base64(b'Zm9v\n'))
+print(binascii.a2b_base64(b'Zm\x009v\n'))
+print(binascii.a2b_base64(b'Zm9v=='))
+print(binascii.a2b_base64(b'Zm9v==='))
+print(binascii.a2b_base64(b'Zm9v===YmFy'))
+
 try:
     print(binascii.a2b_base64(b'abc'))
 except ValueError: