More detailed comments and documentation

This commit is contained in:
topjohnwu 2022-05-11 21:10:17 -07:00 committed by John Wu
parent 9f7a3db8be
commit 575c417403

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@ -2,9 +2,13 @@
using namespace std;
#define APK_SIGNING_BLOCK_MAGIC "APK Sig Block 42"
#define SIGNATURE_SCHEME_V2_MAGIC 0x7109871a
#define EOCD_MAGIC 0x6054b50
// Top-level block container
struct signing_block {
uint64_t len;
uint64_t block_sz;
struct id_value_pair {
uint64_t len;
@ -14,7 +18,7 @@ struct signing_block {
};
} id_value_pair_sequence[0];
uint64_t block_len; // *MUST* be same as len
uint64_t block_sz_; // *MUST* be same as block_sz
char magic[16]; // "APK Sig Block 42"
};
@ -59,78 +63,124 @@ struct v2_signature {
} signer_sequence[0];
};
// The structures above are just for documentation purpose
// The real parsing logic is the following
// End of central directory record
struct EOCD {
uint32_t magic; // 0x6054b50
uint8_t pad[8]; // 8 bytes of irrelevant data
uint32_t central_dir_sz; // size of central directory
uint32_t central_dir_off; // offset of central directory
uint16_t comment_sz; // size of comment
char comment[0];
} __attribute__((packed));
/*
* A v2/v3 signed APK has the format as following
*
* +---------------+
* | zip content |
* +---------------+
* | signing block |
* +---------------+
* | central dir |
* +---------------+
* | EOCD |
* +---------------+
*
* Scan from end of file to find EOCD, and figure our way back to the
* offset of the signing block. Next, directly extract the certificate
* from the v2 signature block.
*
* All structures above are mostly just for documentation purpose.
*
* This method extracts the first certificate of the first signer
* within the APK v2 signature block.
*/
string read_certificate(int fd) {
string certificate;
uint32_t size4;
uint64_t size8, size_of_block;
uint64_t size8;
// Find EOCD
for (int i = 0;; i++) {
unsigned short n;
lseek(fd, -i - 2, SEEK_END);
read(fd, &n, 2);
if (n == i) {
lseek(fd, -22, SEEK_CUR);
read(fd, &size4, 4);
if (size4 == 0x6054b50u) { // central directory end magic
// i is the absolute offset to end of file
uint16_t comment_sz = 0;
lseek(fd, -((off_t) sizeof(comment_sz)) - i, SEEK_END);
read(fd, &comment_sz, sizeof(comment_sz));
if (comment_sz == i) {
// Double check if we actually found the structure
lseek(fd, -((off_t) sizeof(EOCD)), SEEK_CUR);
uint32_t magic = 0;
read(fd, &magic, sizeof(magic));
if (magic == EOCD_MAGIC) {
break;
}
}
if (i == 0xffff) {
return certificate;
// Comments cannot be longer than 0xffff (overflow), abort
return {};
}
}
lseek(fd, 12, SEEK_CUR);
// We are now at EOCD + sizeof(magic)
// Seek and read central_dir_off to find start of central directory
uint32_t central_dir_off = 0;
{
constexpr off_t off = offsetof(EOCD, central_dir_off) - sizeof(EOCD::magic);
lseek(fd, off, SEEK_CUR);
}
read(fd, &central_dir_off, sizeof(central_dir_off));
read(fd, &size4, 0x4);
lseek(fd, (off_t) (size4 - 0x18), SEEK_SET);
read(fd, &size8, 0x8);
char magic[0x10] = {0};
// Next, find the start of the APK signing block
{
constexpr int off = sizeof(signing_block::block_sz_) + sizeof(signing_block::magic);
lseek(fd, (off_t) (central_dir_off - off), SEEK_SET);
}
read(fd, &size8, sizeof(size8)); // size8 = block_sz_
char magic[sizeof(signing_block::magic)] = {0};
read(fd, magic, sizeof(magic));
if (memcmp(magic, "APK Sig Block 42", sizeof(magic)) != 0) {
return certificate;
if (memcmp(magic, APK_SIGNING_BLOCK_MAGIC, sizeof(magic)) != 0) {
// Invalid signing block magic, abort
return {};
}
uint64_t signing_blk_sz = 0;
lseek(fd, (off_t) (central_dir_off - size8 - sizeof(signing_blk_sz)), SEEK_SET);
read(fd, &signing_blk_sz, sizeof(signing_blk_sz));
if (signing_blk_sz != size8) {
// block_sz != block_sz_, invalid signing block format, abort
return {};
}
lseek(fd, (off_t) (size4 - (size8 + 0x8)), SEEK_SET);
read(fd, &size_of_block, 0x8);
if (size_of_block != size8) {
return certificate;
}
// Finally, we are now at the beginning of the id-value pair sequence
for (;;) {
uint32_t id;
uint32_t offset;
read(fd, &size8, 0x8); // sequence length
if (size8 == size_of_block) {
read(fd, &size8, sizeof(size8)); // id-value pair length
if (size8 == signing_blk_sz) {
// Outside of the id-value pair sequence; actually reading block_sz_
break;
}
read(fd, &id, 0x4); // id
offset = 4;
if (id == 0x7109871au) {
read(fd, &size4, 0x4); // signer-sequence length
read(fd, &size4, 0x4); // signer length
read(fd, &size4, 0x4); // signed data length
offset += 0x4 * 3;
uint32_t id;
read(fd, &id, sizeof(id));
if (id == SIGNATURE_SCHEME_V2_MAGIC) {
read(fd, &size4, sizeof(size4)); // signer sequence length
read(fd, &size4, 0x4); // digests-sequence length
lseek(fd, (off_t) (size4), SEEK_CUR);// skip digests
offset += 0x4 + size4;
read(fd, &size4, sizeof(size4)); // signer length
read(fd, &size4, sizeof(size4)); // signed data length
read(fd, &size4, 0x4); // certificates length
read(fd, &size4, 0x4); // certificate length
offset += 0x4 * 2;
read(fd, &size4, sizeof(size4)); // digest sequence length
lseek(fd, (off_t) (size4), SEEK_CUR); // skip all digests
certificate.resize(size4);
read(fd, certificate.data(), size4);
read(fd, &size4, sizeof(size4)); // cert sequence length
read(fd, &size4, sizeof(size4)); // cert length
offset += size4;
string cert;
cert.resize(size4);
read(fd, cert.data(), size4);
return cert;
} else {
// Skip this id-value pair
lseek(fd, (off_t) (size8 - sizeof(id)), SEEK_CUR);
}
lseek(fd, (off_t) (size8 - offset), SEEK_CUR);
}
return certificate;
return {};
}