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package hpke
import (
"crypto/cipher"
"fmt"
)
type encdecContext struct {
// Serialized parameters
suite Suite
sharedSecret []byte
secret []byte
keyScheduleContext []byte
exporterSecret []byte
key []byte
baseNonce []byte
sequenceNumber []byte
// Operational parameters
cipher.AEAD
nonce []byte
}
type (
sealContext struct{ *encdecContext }
openContext struct{ *encdecContext }
)
// Export takes a context string exporterContext and a desired length (in
// bytes), and produces a secret derived from the internal exporter secret
// using the corresponding KDF Expand function. It panics if length is
// greater than 255*N bytes, where N is the size (in bytes) of the KDF's
// output.
func (c *encdecContext) Export(exporterContext []byte, length uint) []byte {
maxLength := uint(255 * c.suite.kdfID.ExtractSize())
if length > maxLength {
panic(fmt.Errorf("output length must be lesser than %v bytes", maxLength))
}
return c.suite.labeledExpand(c.exporterSecret, []byte("sec"),
exporterContext, uint16(length))
}
func (c *encdecContext) Suite() Suite {
return c.suite
}
func (c *encdecContext) calcNonce() []byte {
for i := range c.baseNonce {
c.nonce[i] = c.baseNonce[i] ^ c.sequenceNumber[i]
}
return c.nonce
}
func (c *encdecContext) increment() error {
// tests whether the sequence number is all-ones, which prevents an
// overflow after the increment.
allOnes := byte(0xFF)
for i := range c.sequenceNumber {
allOnes &= c.sequenceNumber[i]
}
if allOnes == byte(0xFF) {
return ErrAEADSeqOverflows
}
// performs an increment by 1 and verifies whether the sequence overflows.
carry := uint(1)
for i := len(c.sequenceNumber) - 1; i >= 0; i-- {
sum := uint(c.sequenceNumber[i]) + carry
carry = sum >> 8
c.sequenceNumber[i] = byte(sum & 0xFF)
}
if carry != 0 {
return ErrAEADSeqOverflows
}
return nil
}
func (c *sealContext) Seal(pt, aad []byte) ([]byte, error) {
ct := c.AEAD.Seal(nil, c.calcNonce(), pt, aad)
err := c.increment()
if err != nil {
for i := range ct {
ct[i] = 0
}
return nil, err
}
return ct, nil
}
func (c *openContext) Open(ct, aad []byte) ([]byte, error) {
pt, err := c.AEAD.Open(nil, c.calcNonce(), ct, aad)
if err != nil {
return nil, err
}
err = c.increment()
if err != nil {
for i := range pt {
pt[i] = 0
}
return nil, err
}
return pt, nil
}
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